[function.objects]

# 22 General utilities library [[utilities]](./#utilities)

## 22.10 Function objects [function.objects]

### [22.10.1](#general) General [[function.objects.general]](function.objects.general)

[1](#general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11215)

A [*function object type*](#def:function_object,type "22.10.1 General [function.objects.general]") is an object
type ([[basic.types.general]](basic.types.general#term.object.type "6.9.1 General")) that can be the type of the[*postfix-expression*](expr.post.general#nt:postfix-expression "7.6.1.1 General [expr.post.general]") in a function call ([[expr.call]](expr.call "7.6.1.3 Function call"), [[over.match.call]](over.match.call "12.2.2.2 Function call syntax"))[.](#general-1.sentence-1)[193](#footnote-193 "Such a type is a function pointer or a class type which has a member operator() or a class type which has a conversion to a pointer to function.")

A [*function object*](#def:function_object "22.10.1 General [function.objects.general]") is an
object of a function object type[.](#general-1.sentence-2)

In the places where one would expect to pass a
pointer to a function to an algorithmic template ([[algorithms]](algorithms "26 Algorithms library")), the
interface is specified to accept a function object[.](#general-1.sentence-3)

This not only makes
algorithmic templates work with pointers to functions, but also enables them to
work with arbitrary function objects[.](#general-1.sentence-4)

[193)](#footnote-193)[193)](#footnoteref-193)

Such a type is a function
pointer or a class type which has a member operator() or a class type
which has a conversion to a pointer to function[.](#footnote-193.sentence-1)

### [22.10.2](#functional.syn) Header <functional> synopsis [[functional.syn]](functional.syn)

[🔗](#header:%3cfunctional%3e)

namespace std {// [[func.invoke]](#func.invoke "22.10.5 invoke functions"), invoketemplate<class F, class... Args>constexpr invoke_result_t<F, Args...> invoke(F&& f, Args&&... args) // freestandingnoexcept(is_nothrow_invocable_v<F, Args...>); template<class R, class F, class... Args>constexpr R invoke_r(F&& f, Args&&... args) // freestandingnoexcept(is_nothrow_invocable_r_v<R, F, Args...>); // [[refwrap]](#refwrap "22.10.6 Class template reference_­wrapper"), reference_wrappertemplate<class T> class reference_wrapper; // freestandingtemplate<class T> constexpr reference_wrapper<T> ref(T&) noexcept; // freestandingtemplate<class T> constexpr reference_wrapper<const T> cref(const T&) noexcept; // freestandingtemplate<class T> void ref(const T&&) = delete; // freestandingtemplate<class T> void cref(const T&&) = delete; // freestandingtemplate<class T>constexpr reference_wrapper<T> ref(reference_wrapper<T>) noexcept; // freestandingtemplate<class T>constexpr reference_wrapper<const T> cref(reference_wrapper<T>) noexcept; // freestanding// [[refwrap.common.ref]](#refwrap.common.ref "22.10.6.8 common_­reference related specializations"), common_reference related specializationstemplate<class R, class T, template<class> class RQual, template<class> class TQual>requires *see below*struct basic_common_reference<R, T, RQual, TQual>; template<class T, class R, template<class> class TQual, template<class> class RQual>requires *see below*struct basic_common_reference<T, R, TQual, RQual>; // [[arithmetic.operations]](#arithmetic.operations "22.10.7 Arithmetic operations"), arithmetic operationstemplate<class T = void> struct plus; // freestandingtemplate<class T = void> struct minus; // freestandingtemplate<class T = void> struct multiplies; // freestandingtemplate<class T = void> struct divides; // freestandingtemplate<class T = void> struct modulus; // freestandingtemplate<class T = void> struct negate; // freestandingtemplate<> struct plus<void>; // freestandingtemplate<> struct minus<void>; // freestandingtemplate<> struct multiplies<void>; // freestandingtemplate<> struct divides<void>; // freestandingtemplate<> struct modulus<void>; // freestandingtemplate<> struct negate<void>; // freestanding// [[comparisons]](#comparisons "22.10.8 Comparisons"), comparisonstemplate<class T = void> struct equal_to; // freestandingtemplate<class T = void> struct not_equal_to; // freestandingtemplate<class T = void> struct greater; // freestandingtemplate<class T = void> struct less; // freestandingtemplate<class T = void> struct greater_equal; // freestandingtemplate<class T = void> struct less_equal; // freestandingtemplate<> struct equal_to<void>; // freestandingtemplate<> struct not_equal_to<void>; // freestandingtemplate<> struct greater<void>; // freestandingtemplate<> struct less<void>; // freestandingtemplate<> struct greater_equal<void>; // freestandingtemplate<> struct less_equal<void>; // freestanding// [[comparisons.three.way]](#comparisons.three.way "22.10.8.8 Class compare_­three_­way"), class compare_three_waystruct compare_three_way; // freestanding// [[logical.operations]](#logical.operations "22.10.10 Logical operations"), logical operationstemplate<class T = void> struct logical_and; // freestandingtemplate<class T = void> struct logical_or; // freestandingtemplate<class T = void> struct logical_not; // freestandingtemplate<> struct logical_and<void>; // freestandingtemplate<> struct logical_or<void>; // freestandingtemplate<> struct logical_not<void>; // freestanding// [[bitwise.operations]](#bitwise.operations "22.10.11 Bitwise operations"), bitwise operationstemplate<class T = void> struct bit_and; // freestandingtemplate<class T = void> struct bit_or; // freestandingtemplate<class T = void> struct bit_xor; // freestandingtemplate<class T = void> struct bit_not; // freestandingtemplate<> struct bit_and<void>; // freestandingtemplate<> struct bit_or<void>; // freestandingtemplate<> struct bit_xor<void>; // freestandingtemplate<> struct bit_not<void>; // freestanding// [[func.identity]](#func.identity "22.10.12 Class identity"), identitystruct identity; // freestanding// [[func.not.fn]](#func.not.fn "22.10.13 Function template not_­fn"), function template not_fntemplate<class F> constexpr *unspecified* not_fn(F&& f); // freestandingtemplate<auto f> constexpr *unspecified* not_fn() noexcept; // freestanding// [[func.bind.partial]](#func.bind.partial "22.10.14 Function templates bind_­front and bind_­back"), function templates bind_front and bind_backtemplate<class F, class... Args>constexpr *unspecified* bind_front(F&&, Args&&...); // freestandingtemplate<auto f, class... Args>constexpr *unspecified* bind_front(Args&&...); // freestandingtemplate<class F, class... Args>constexpr *unspecified* bind_back(F&&, Args&&...); // freestandingtemplate<auto f, class... Args>constexpr *unspecified* bind_back(Args&&...); // freestanding// [[func.bind]](#func.bind "22.10.15 Function object binders"), bindtemplate<class T> struct is_bind_expression; // freestandingtemplate<class T>constexpr bool [is_bind_expression_v](#lib:is_bind_expression_v "22.10.2 Header <functional> synopsis [functional.syn]") = // freestanding is_bind_expression<T>::value; template<class T> struct is_placeholder; // freestandingtemplate<class T>constexpr int [is_placeholder_v](#lib:is_placeholder_v "22.10.2 Header <functional> synopsis [functional.syn]") = // freestanding is_placeholder<T>::value; template<class F, class... BoundArgs>constexpr *unspecified* bind(F&&, BoundArgs&&...); // freestandingtemplate<class R, class F, class... BoundArgs>constexpr *unspecified* bind(F&&, BoundArgs&&...); // freestandingnamespace placeholders {// *M* is the implementation-defined number of placeholders*see below* _1; // freestanding*see below* _2; // freestanding ⋮ *see below* _*M*; // freestanding}// [[func.memfn]](#func.memfn "22.10.16 Function template mem_­fn"), member function adaptorstemplate<class R, class T>constexpr *unspecified* mem_fn(R T::*) noexcept; // freestanding// [[func.wrap]](#func.wrap "22.10.17 Polymorphic function wrappers"), polymorphic function wrappers// [[func.wrap.badcall]](#func.wrap.badcall "22.10.17.2 Class bad_­function_­call"), class bad_function_callclass bad_function_call; // [[func.wrap.func]](#func.wrap.func "22.10.17.3 Class template function"), class template functiontemplate<class> class function; // *not defined*template<class R, class... ArgTypes> class function<R(ArgTypes...)>; // [[func.wrap.func.alg]](#func.wrap.func.alg "22.10.17.3.8 Specialized algorithms"), function specialized algorithmstemplate<class R, class... ArgTypes>void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&) noexcept; // [[func.wrap.func.nullptr]](#func.wrap.func.nullptr "22.10.17.3.7 Null pointer comparison operator functions"), function null pointer comparison operator functionstemplate<class R, class... ArgTypes>bool operator==(const function<R(ArgTypes...)>&, nullptr_t) noexcept; // [[func.wrap.move]](#func.wrap.move "22.10.17.4 Move-only wrapper"), move-only wrappertemplate<class... S> class move_only_function; // *not defined*template<class R, class... ArgTypes>class move_only_function<R(ArgTypes...) cv *ref* noexcept(*noex*)>; // *see below*// [[func.wrap.copy]](#func.wrap.copy "22.10.17.5 Copyable wrapper"), copyable wrappertemplate<class... S> class copyable_function; // *not defined*template<class R, class... ArgTypes>class copyable_function<R(ArgTypes...) cv *ref* noexcept(*noex*)>; // *see below*// [[func.wrap.ref]](#func.wrap.ref "22.10.17.6 Non-owning wrapper"), non-owning wrappertemplate<class... S> class function_ref; // freestanding, *not defined*template<class R, class... ArgTypes>class function_ref<R(ArgTypes...) cv noexcept(*noex*)>; // freestanding, *see below*// [[func.search]](#func.search "22.10.18 Searchers"), searcherstemplate<class ForwardIterator1, class BinaryPredicate = equal_to<>>class default_searcher; // freestandingtemplate<class RandomAccessIterator, class Hash = hash<typename iterator_traits<RandomAccessIterator>::value_type>, class BinaryPredicate = equal_to<>>class boyer_moore_searcher; template<class RandomAccessIterator, class Hash = hash<typename iterator_traits<RandomAccessIterator>::value_type>, class BinaryPredicate = equal_to<>>class boyer_moore_horspool_searcher; // [[unord.hash]](#unord.hash "22.10.19 Class template hash"), class template hashtemplate<class T>struct hash; // freestandingnamespace ranges {// [[range.cmp]](#range.cmp "22.10.9 Concept-constrained comparisons"), concept-constrained comparisonsstruct equal_to; // freestandingstruct not_equal_to; // freestandingstruct greater; // freestandingstruct less; // freestandingstruct greater_equal; // freestandingstruct less_equal; // freestanding}template<class Fn, class... Args>concept [*callable*](#concept:callable "22.10.2 Header <functional> synopsis [functional.syn]") = // *exposition only*requires (Fn&& fn, Args&&... args) { std::forward<Fn>(fn)(std::forward<Args>(args)...); }; template<class Fn, class... Args>concept [*nothrow-callable*](#concept:nothrow-callable "22.10.2 Header <functional> synopsis [functional.syn]") = // *exposition only*[*callable*](#concept:callable "22.10.2 Header <functional> synopsis [functional.syn]")<Fn, Args...> &&requires (Fn&& fn, Args&&... args) {{ std::forward<Fn>(fn)(std::forward<Args>(args)...) } noexcept; }; template<class Fn, class... Args>using *call-result-t* = decltype(declval<Fn>()(declval<Args>()...)); // *exposition only*template<const auto& T>using *decayed-typeof* = decltype(auto(T)); // *exposition only*}

[1](#functional.syn-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11443)

[*Example [1](#functional.syn-example-1)*:

If a C++ program wants to have a by-element addition of two vectors a and b containing double and put the result into a,
it can do:transform(a.begin(), a.end(), b.begin(), a.begin(), plus<double>());

— *end example*]

[2](#functional.syn-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11453)

[*Example [2](#functional.syn-example-2)*:

To negate every element of a:

transform(a.begin(), a.end(), a.begin(), negate<double>()); — *end example*]

### [22.10.3](#func.def) Definitions [[func.def]](func.def)

[1](#func.def-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11465)

The following definitions apply to this Clause:

[2](#func.def-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11468)

A [*call signature*](#def:call_signature "22.10.3 Definitions [func.def]") is the name of a return type followed by a
parenthesized comma-separated list of zero or more argument types[.](#func.def-2.sentence-1)

[3](#func.def-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11472)

A [*callable type*](#def:type,callable "22.10.3 Definitions [func.def]") is a function object type ([function.objects]) or a pointer to member[.](#func.def-3.sentence-1)

[4](#func.def-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11475)

A [*callable object*](#def:object,callable "22.10.3 Definitions [func.def]") is an object of a callable type[.](#func.def-4.sentence-1)

[5](#func.def-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11478)

A [*call wrapper type*](#def:call_wrapper,type "22.10.3 Definitions [func.def]") is a type that holds a callable object
and supports a call operation that forwards to that object[.](#func.def-5.sentence-1)

[6](#func.def-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11482)

A [*call wrapper*](#def:call_wrapper "22.10.3 Definitions [func.def]") is an object of a call wrapper type[.](#func.def-6.sentence-1)

[7](#func.def-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11485)

A [*target object*](#def:target_object "22.10.3 Definitions [func.def]") is the callable object held by a call wrapper[.](#func.def-7.sentence-1)

[8](#func.def-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11488)

A call wrapper type may additionally hold
a sequence of objects and references
that may be passed as arguments to the target object[.](#func.def-8.sentence-1)

These entities are collectively referred to
as [*bound argument entities*](#def:bound_argument_entity "22.10.3 Definitions [func.def]")[.](#func.def-8.sentence-2)

[9](#func.def-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11495)

The target object and bound argument entities of the call wrapper are
collectively referred to as [*state entities*](#def:state_entity "22.10.3 Definitions [func.def]")[.](#func.def-9.sentence-1)

### [22.10.4](#func.require) Requirements [[func.require]](func.require)

[1](#func.require-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11501)

Define *INVOKE*(f, t1, t2, …, tN) as follows:

- [(1.1)](#func.require-1.1)

(t1.*f)(t2, …, tN) when f is a pointer to a
member function of a class T andis_same_v<T, remove_cvref_t<decltype(t1)>> ||is_base_of_v<T, remove_cvref_t<decltype(t1)>> is true;

- [(1.2)](#func.require-1.2)

(t1.get().*f)(t2, …, tN) when f is a pointer to a
member function of a class T and remove_cvref_t<decltype(t1)> is a specialization of reference_wrapper;

- [(1.3)](#func.require-1.3)

((*t1).*f)(t2, …, tN) when f is a pointer to a
member function of a class T and t1 does not satisfy the previous two items;

- [(1.4)](#func.require-1.4)

t1.*f when N=1 and f is a pointer to
data member of a class T andis_same_v<T, remove_cvref_t<decltype(t1)>> ||is_base_of_v<T, remove_cvref_t<decltype(t1)>> is true;

- [(1.5)](#func.require-1.5)

t1.get().*f when N=1 and f is a pointer to
data member of a class T and remove_cvref_t<decltype(t1)> is a specialization of reference_wrapper;

- [(1.6)](#func.require-1.6)

(*t1).*f when N=1 and f is a pointer to
data member of a class T and t1 does not satisfy the previous two items;

- [(1.7)](#func.require-1.7)

f(t1, t2, …, tN) in all other cases[.](#func.require-1.sentence-1)

[2](#func.require-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11537)

Define *INVOKE*<R>(f, t1, t2, …, tN) asstatic_cast<void>(*INVOKE*(f, t1, t2, …, tN)) if R is cv void, otherwise*INVOKE*(f, t1, t2, …, tN) implicitly converted
to R[.](#func.require-2.sentence-1)

Ifreference_converts_from_temporary_v<R, decltype(*INVOKE*(f, t1, t2, …, tN))> is true,*INVOKE*<R>(f, t1, t2, …, tN) is ill-formed[.](#func.require-2.sentence-2)

[3](#func.require-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11550)

Every call wrapper ([[func.def]](#func.def "22.10.3 Definitions")) meets the [*Cpp17MoveConstructible*](utility.arg.requirements#:Cpp17MoveConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") and [*Cpp17Destructible*](utility.arg.requirements#:Cpp17Destructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements[.](#func.require-3.sentence-1)

An [*argument forwarding call wrapper*](#def:argument_forwarding_call_wrapper "22.10.4 Requirements [func.require]") is a
call wrapper that can be called with an arbitrary argument list
and delivers the arguments to the target object as references[.](#func.require-3.sentence-2)

This forwarding step delivers rvalue arguments as rvalue references
and lvalue arguments as lvalue references[.](#func.require-3.sentence-3)

[*Note [1](#func.require-note-1)*:

In a typical implementation, argument forwarding call wrappers have
an overloaded function call operator of the formtemplate<class... UnBoundArgs>constexpr R operator()(UnBoundArgs&&... unbound_args) *cv-qual*;

— *end note*]

[4](#func.require-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11570)

A [*perfect forwarding call wrapper*](#def:call_wrapper,perfect_forwarding "22.10.4 Requirements [func.require]") is
an argument forwarding call wrapper
that forwards its state entities to the underlying call expression[.](#func.require-4.sentence-1)

This forwarding step delivers a state entity of type T as cv T& when the call is performed on an lvalue of the call wrapper type and
as cv T&& otherwise,
where cv represents the cv-qualifiers of the call wrapper and
where cv shall be neither volatile nor const volatile[.](#func.require-4.sentence-2)

[5](#func.require-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11582)

A [*call pattern*](#def:call_pattern "22.10.4 Requirements [func.require]") defines the semantics of invoking
a perfect forwarding call wrapper[.](#func.require-5.sentence-1)

A postfix call performed on a perfect forwarding call wrapper is
expression-equivalent ([[defns.expression.equivalent]](defns.expression.equivalent "3.22 expression-equivalent")) to
an expression e determined from its call pattern cp by replacing all occurrences
of the arguments of the call wrapper and its state entities
with references as described in the corresponding forwarding steps[.](#func.require-5.sentence-2)

[6](#func.require-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11592)

A [*simple call wrapper*](#def:simple_call_wrapper "22.10.4 Requirements [func.require]") is a perfect forwarding call wrapper that meets
the [*Cpp17CopyConstructible*](utility.arg.requirements#:Cpp17CopyConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") and [*Cpp17CopyAssignable*](utility.arg.requirements#:Cpp17CopyAssignable "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements
and whose copy constructor, move constructor, and assignment operators
are constexpr functions that do not throw exceptions[.](#func.require-6.sentence-1)

[7](#func.require-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11599)

The copy/move constructor of an argument forwarding call wrapper has
the same apparent semantics
as if memberwise copy/move of its state entities
were performed ([[class.copy.ctor]](class.copy.ctor "11.4.5.3 Copy/move constructors"))[.](#func.require-7.sentence-1)

[*Note [2](#func.require-note-2)*:

This implies that each of the copy/move constructors has
the same exception-specification as
the corresponding implicit definition and is declared as constexpr if the corresponding implicit definition would be considered to be constexpr[.](#func.require-7.sentence-2)

— *end note*]

[8](#func.require-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11611)

Argument forwarding call wrappers returned by
a given standard library function template have the same type
if the types of their corresponding state entities are the same[.](#func.require-8.sentence-1)

### [22.10.5](#func.invoke) invoke functions [[func.invoke]](func.invoke)

[🔗](#lib:invoke)

`template<class F, class... Args>
  constexpr invoke_result_t<F, Args...> invoke(F&& f, Args&&... args)
    noexcept(is_nothrow_invocable_v<F, Args...>);
`

[1](#func.invoke-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11626)

*Constraints*: is_invocable_v<F, Args...> is true[.](#func.invoke-1.sentence-1)

[2](#func.invoke-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11630)

*Returns*: *INVOKE*(std​::​forward<F>(f), std​::​forward<Args>(args)...) ([[func.require]](#func.require "22.10.4 Requirements"))[.](#func.invoke-2.sentence-1)

[🔗](#lib:invoke_r)

`template<class R, class F, class... Args>
  constexpr R invoke_r(F&& f, Args&&... args)
    noexcept(is_nothrow_invocable_r_v<R, F, Args...>);
`

[3](#func.invoke-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11643)

*Constraints*: is_invocable_r_v<R, F, Args...> is true[.](#func.invoke-3.sentence-1)

[4](#func.invoke-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11647)

*Returns*: *INVOKE*<R>(std​::​forward<F>(f), std​::​forward<Args>(args)...) ([[func.require]](#func.require "22.10.4 Requirements"))[.](#func.invoke-4.sentence-1)

### [22.10.6](#refwrap) Class template reference_wrapper [[refwrap]](refwrap)

#### [22.10.6.1](#refwrap.general) General [[refwrap.general]](refwrap.general)

[🔗](#lib:reference_wrapper)

namespace std {template<class T> class reference_wrapper {public:// typesusing type = T; // [[refwrap.const]](#refwrap.const "22.10.6.2 Constructors"), constructorstemplate<class U>constexpr reference_wrapper(U&&) noexcept(*see below*); constexpr reference_wrapper(const reference_wrapper& x) noexcept; // [[refwrap.assign]](#refwrap.assign "22.10.6.3 Assignment"), assignmentconstexpr reference_wrapper& operator=(const reference_wrapper& x) noexcept; // [[refwrap.access]](#refwrap.access "22.10.6.4 Access"), accessconstexpr operator T& () const noexcept; constexpr T& get() const noexcept; // [[refwrap.invoke]](#refwrap.invoke "22.10.6.5 Invocation"), invocationtemplate<class... ArgTypes>constexpr invoke_result_t<T&, ArgTypes...> operator()(ArgTypes&&...) constnoexcept(is_nothrow_invocable_v<T&, ArgTypes...>); // [[refwrap.comparisons]](#refwrap.comparisons "22.10.6.6 Comparisons"), comparisonsfriend constexpr bool operator==(reference_wrapper, reference_wrapper); friend constexpr bool operator==(reference_wrapper, const T&); friend constexpr bool operator==(reference_wrapper, reference_wrapper<const T>); friend constexpr auto operator<=>(reference_wrapper, reference_wrapper); friend constexpr auto operator<=>(reference_wrapper, const T&); friend constexpr auto operator<=>(reference_wrapper, reference_wrapper<const T>); }; template<class T> reference_wrapper(T&) -> reference_wrapper<T>;}

[1](#refwrap.general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11697)

reference_wrapper<T> is a [*Cpp17CopyConstructible*](utility.arg.requirements#:Cpp17CopyConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") and [*Cpp17CopyAssignable*](utility.arg.requirements#:Cpp17CopyAssignable "16.4.4.2 Template argument requirements [utility.arg.requirements]") wrapper
around a reference to an object or function of type T[.](#refwrap.general-1.sentence-1)

[2](#refwrap.general-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11701)

reference_wrapper<T> is
a trivially copyable type ([[basic.types.general]](basic.types.general#term.trivially.copyable.type "6.9.1 General"))[.](#refwrap.general-2.sentence-1)

[3](#refwrap.general-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11705)

The template parameter T of reference_wrapper may be an incomplete type[.](#refwrap.general-3.sentence-1)

[*Note [1](#refwrap.general-note-1)*:

Using the comparison operators described in [[refwrap.comparisons]](#refwrap.comparisons "22.10.6.6 Comparisons") with T being an incomplete type
can lead to an ill-formed program
with no diagnostic required ([[temp.point]](temp.point "13.8.4.1 Point of instantiation"), [[temp.constr.atomic]](temp.constr.atomic "13.5.2.3 Atomic constraints"))[.](#refwrap.general-3.sentence-2)

— *end note*]

#### [22.10.6.2](#refwrap.const) Constructors [[refwrap.const]](refwrap.const)

[🔗](#lib:reference_wrapper,constructor)

`template<class U>
  constexpr reference_wrapper(U&& u) noexcept(see below);
`

[1](#refwrap.const-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11724)

Let *FUN* denote the exposition-only functionsvoid *FUN*(T&) noexcept;void *FUN*(T&&) = delete;

[2](#refwrap.const-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11731)

*Constraints*: The expression *FUN*(declval<U>()) is well-formed andis_same_v<remove_cvref_t<U>, reference_wrapper> is false[.](#refwrap.const-2.sentence-1)

[3](#refwrap.const-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11736)

*Effects*: Creates a variable r as if by T& r = std​::​forward<U>(u),
then constructs a reference_wrapper object
that stores a reference to r[.](#refwrap.const-3.sentence-1)

[4](#refwrap.const-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11743)

*Remarks*: The exception specification is equivalent tonoexcept(*FUN*(declval<U>()))[.](#refwrap.const-4.sentence-1)

[🔗](#lib:reference_wrapper,constructor_)

`constexpr reference_wrapper(const reference_wrapper& x) noexcept;
`

[5](#refwrap.const-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11755)

*Effects*: Constructs a reference_wrapper object that
stores a reference to x.get()[.](#refwrap.const-5.sentence-1)

#### [22.10.6.3](#refwrap.assign) Assignment [[refwrap.assign]](refwrap.assign)

[🔗](#lib:operator=,reference_wrapper)

`constexpr reference_wrapper& operator=(const reference_wrapper& x) noexcept;
`

[1](#refwrap.assign-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11769)

*Postconditions*: *this stores a reference to x.get()[.](#refwrap.assign-1.sentence-1)

#### [22.10.6.4](#refwrap.access) Access [[refwrap.access]](refwrap.access)

[🔗](#lib:operator_T&,reference_wrapper)

`constexpr operator T& () const noexcept;
`

[1](#refwrap.access-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11782)

*Returns*: The stored reference[.](#refwrap.access-1.sentence-1)

[🔗](#lib:get,reference_wrapper)

`constexpr T& get() const noexcept;
`

[2](#refwrap.access-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11793)

*Returns*: The stored reference[.](#refwrap.access-2.sentence-1)

#### [22.10.6.5](#refwrap.invoke) Invocation [[refwrap.invoke]](refwrap.invoke)

[🔗](#lib:operator(),reference_wrapper)

`template<class... ArgTypes>
  constexpr invoke_result_t<T&, ArgTypes...>
    operator()(ArgTypes&&... args) const noexcept(is_nothrow_invocable_v<T&, ArgTypes...>);
`

[1](#refwrap.invoke-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11808)

*Mandates*: T is a complete type[.](#refwrap.invoke-1.sentence-1)

[2](#refwrap.invoke-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11812)

*Returns*: *INVOKE*(get(), std​::​forward<ArgTypes>(args)...) ([[func.require]](#func.require "22.10.4 Requirements"))[.](#refwrap.invoke-2.sentence-1)

#### [22.10.6.6](#refwrap.comparisons) Comparisons [[refwrap.comparisons]](refwrap.comparisons)

[🔗](#refwrap.comparisons-itemdecl:1)

`friend constexpr bool operator==(reference_wrapper x, reference_wrapper y);
`

[1](#refwrap.comparisons-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11824)

*Constraints*: The expression x.get() == y.get() is well-formed and
its result is convertible to bool[.](#refwrap.comparisons-1.sentence-1)

[2](#refwrap.comparisons-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11829)

*Returns*: x.get() == y.get()[.](#refwrap.comparisons-2.sentence-1)

[🔗](#refwrap.comparisons-itemdecl:2)

`friend constexpr bool operator==(reference_wrapper x, const T& y);
`

[3](#refwrap.comparisons-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11839)

*Constraints*: The expression x.get() == y is well-formed and
its result is convertible to bool[.](#refwrap.comparisons-3.sentence-1)

[4](#refwrap.comparisons-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11844)

*Returns*: x.get() == y[.](#refwrap.comparisons-4.sentence-1)

[🔗](#refwrap.comparisons-itemdecl:3)

`friend constexpr bool operator==(reference_wrapper x, reference_wrapper<const T> y);
`

[5](#refwrap.comparisons-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11854)

*Constraints*: is_const_v<T> is false and
the expression x.get() == y.get() is well-formed and
its result is convertible to bool[.](#refwrap.comparisons-5.sentence-1)

[6](#refwrap.comparisons-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11860)

*Returns*: x.get() == y.get()[.](#refwrap.comparisons-6.sentence-1)

[🔗](#refwrap.comparisons-itemdecl:4)

`friend constexpr auto operator<=>(reference_wrapper x, reference_wrapper y);
`

[7](#refwrap.comparisons-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11870)

*Constraints*: The expression *synth-three-way*(x.get(), y.get()) is well-formed[.](#refwrap.comparisons-7.sentence-1)

[8](#refwrap.comparisons-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11875)

*Returns*: *synth-three-way*(x.get(), y.get())[.](#refwrap.comparisons-8.sentence-1)

[🔗](#refwrap.comparisons-itemdecl:5)

`friend constexpr auto operator<=>(reference_wrapper x, const T& y);
`

[9](#refwrap.comparisons-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11885)

*Constraints*: The expression *synth-three-way*(x.get(), y) is well-formed[.](#refwrap.comparisons-9.sentence-1)

[10](#refwrap.comparisons-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11890)

*Returns*: *synth-three-way*(x.get(), y)[.](#refwrap.comparisons-10.sentence-1)

[🔗](#refwrap.comparisons-itemdecl:6)

`friend constexpr auto operator<=>(reference_wrapper x, reference_wrapper<const T> y);
`

[11](#refwrap.comparisons-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11900)

*Constraints*: is_const_v<T> is false[.](#refwrap.comparisons-11.sentence-1)

The expression *synth-three-way*(x.get(), y.get()) is well-formed[.](#refwrap.comparisons-11.sentence-2)

[12](#refwrap.comparisons-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11906)

*Returns*: *synth-three-way*(x.get(), y.get())[.](#refwrap.comparisons-12.sentence-1)

#### [22.10.6.7](#refwrap.helpers) Helper functions [[refwrap.helpers]](refwrap.helpers)

[1](#refwrap.helpers-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11913)

The template parameter T of
the following ref and cref function templates
may be an incomplete type[.](#refwrap.helpers-1.sentence-1)

[🔗](#lib:ref,reference_wrapper)

`template<class T> constexpr reference_wrapper<T> ref(T& t) noexcept;
`

[2](#refwrap.helpers-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11924)

*Returns*: reference_wrapper<T>(t)[.](#refwrap.helpers-2.sentence-1)

[🔗](#lib:ref,reference_wrapper_)

`template<class T> constexpr reference_wrapper<T> ref(reference_wrapper<T> t) noexcept;
`

[3](#refwrap.helpers-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11935)

*Returns*: t[.](#refwrap.helpers-3.sentence-1)

[🔗](#lib:cref,reference_wrapper)

`template<class T> constexpr reference_wrapper<const T> cref(const T& t) noexcept;
`

[4](#refwrap.helpers-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11946)

*Returns*: reference_wrapper<const T>(t)[.](#refwrap.helpers-4.sentence-1)

[🔗](#lib:cref,reference_wrapper_)

`template<class T> constexpr reference_wrapper<const T> cref(reference_wrapper<T> t) noexcept;
`

[5](#refwrap.helpers-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11957)

*Returns*: t[.](#refwrap.helpers-5.sentence-1)

#### [22.10.6.8](#refwrap.common.ref) common_reference related specializations [[refwrap.common.ref]](refwrap.common.ref)

[🔗](#lib:basic_common_reference)

namespace std {template<class T>constexpr bool *is-ref-wrapper* = false; // *exposition only*template<class T>constexpr bool *is-ref-wrapper*<reference_wrapper<T>> = true; template<class R, class T, class RQ, class TQ>concept [*ref-wrap-common-reference-exists-with*](#concept:ref-wrap-common-reference-exists-with "22.10.6.8 common_­reference related specializations [refwrap.common.ref]") = // *exposition only**is-ref-wrapper*<R> &&requires { typename common_reference_t<typename R::type&, TQ>; } &&[convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<RQ, common_reference_t<typename R::type&, TQ>>; template<class R, class T, template<class> class RQual, template<class> class TQual>requires ([*ref-wrap-common-reference-exists-with*](#concept:ref-wrap-common-reference-exists-with "22.10.6.8 common_­reference related specializations [refwrap.common.ref]")<R, T, RQual<R>, TQual<T>> &&![*ref-wrap-common-reference-exists-with*](#concept:ref-wrap-common-reference-exists-with "22.10.6.8 common_­reference related specializations [refwrap.common.ref]")<T, R, TQual<T>, RQual<R>>)struct basic_common_reference<R, T, RQual, TQual> {using type = common_reference_t<typename R::type&, TQual<T>>; }; template<class T, class R, template<class> class TQual, template<class> class RQual>requires ([*ref-wrap-common-reference-exists-with*](#concept:ref-wrap-common-reference-exists-with "22.10.6.8 common_­reference related specializations [refwrap.common.ref]")<R, T, RQual<R>, TQual<T>> &&![*ref-wrap-common-reference-exists-with*](#concept:ref-wrap-common-reference-exists-with "22.10.6.8 common_­reference related specializations [refwrap.common.ref]")<T, R, TQual<T>, RQual<R>>)struct basic_common_reference<T, R, TQual, RQual> {using type = common_reference_t<typename R::type&, TQual<T>>; };}

### [22.10.7](#arithmetic.operations) Arithmetic operations [[arithmetic.operations]](arithmetic.operations)

#### [22.10.7.1](#arithmetic.operations.general) General [[arithmetic.operations.general]](arithmetic.operations.general)

[1](#arithmetic.operations.general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11999)

The library provides basic function object classes for all of the arithmetic
operators in the language ([[expr.mul]](expr.mul "7.6.5 Multiplicative operators"), [[expr.add]](expr.add "7.6.6 Additive operators"))[.](#arithmetic.operations.general-1.sentence-1)

#### [22.10.7.2](#arithmetic.operations.plus) Class template plus [[arithmetic.operations.plus]](arithmetic.operations.plus)

[🔗](#lib:plus)

`template<class T = void> struct plus {
  constexpr T operator()(const T& x, const T& y) const;
};
`

[🔗](#lib:operator(),plus)

`constexpr T operator()(const T& x, const T& y) const;
`

[1](#arithmetic.operations.plus-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12018)

*Returns*: x + y[.](#arithmetic.operations.plus-1.sentence-1)

[🔗](#lib:plus%3c%3e)

`template<> struct plus<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) + std::forward<U>(u));

  using is_transparent = unspecified;
};
`

[🔗](#lib:operator(),plus%3c%3e)

`template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) + std::forward<U>(u));
`

[2](#arithmetic.operations.plus-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12040)

*Returns*: std​::​forward<T>(t) + std​::​forward<U>(u)[.](#arithmetic.operations.plus-2.sentence-1)

#### [22.10.7.3](#arithmetic.operations.minus) Class template minus [[arithmetic.operations.minus]](arithmetic.operations.minus)

[🔗](#lib:minus)

`template<class T = void> struct minus {
  constexpr T operator()(const T& x, const T& y) const;
};
`

[🔗](#lib:operator(),minus)

`constexpr T operator()(const T& x, const T& y) const;
`

[1](#arithmetic.operations.minus-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12060)

*Returns*: x - y[.](#arithmetic.operations.minus-1.sentence-1)

[🔗](#lib:minus%3c%3e)

`template<> struct minus<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) - std::forward<U>(u));

  using is_transparent = unspecified;
};
`

[🔗](#lib:operator(),minus%3c%3e)

`template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) - std::forward<U>(u));
`

[2](#arithmetic.operations.minus-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12082)

*Returns*: std​::​forward<T>(t) - std​::​forward<U>(u)[.](#arithmetic.operations.minus-2.sentence-1)

#### [22.10.7.4](#arithmetic.operations.multiplies) Class template multiplies [[arithmetic.operations.multiplies]](arithmetic.operations.multiplies)

[🔗](#lib:multiplies)

`template<class T = void> struct multiplies {
  constexpr T operator()(const T& x, const T& y) const;
};
`

[🔗](#lib:operator(),multiplies)

`constexpr T operator()(const T& x, const T& y) const;
`

[1](#arithmetic.operations.multiplies-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12102)

*Returns*: x * y[.](#arithmetic.operations.multiplies-1.sentence-1)

[🔗](#lib:multiplies%3c%3e)

`template<> struct multiplies<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) * std::forward<U>(u));

  using is_transparent = unspecified;
};
`

[🔗](#lib:operator(),multiplies%3c%3e)

`template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) * std::forward<U>(u));
`

[2](#arithmetic.operations.multiplies-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12124)

*Returns*: std​::​forward<T>(t) * std​::​forward<U>(u)[.](#arithmetic.operations.multiplies-2.sentence-1)

#### [22.10.7.5](#arithmetic.operations.divides) Class template divides [[arithmetic.operations.divides]](arithmetic.operations.divides)

[🔗](#lib:divides)

`template<class T = void> struct divides {
  constexpr T operator()(const T& x, const T& y) const;
};
`

[🔗](#lib:operator(),divides)

`constexpr T operator()(const T& x, const T& y) const;
`

[1](#arithmetic.operations.divides-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12144)

*Returns*: x / y[.](#arithmetic.operations.divides-1.sentence-1)

[🔗](#lib:divides%3c%3e)

`template<> struct divides<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) / std::forward<U>(u));

  using is_transparent = unspecified;
};
`

[🔗](#lib:operator(),divides%3c%3e)

`template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) / std::forward<U>(u));
`

[2](#arithmetic.operations.divides-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12166)

*Returns*: std​::​forward<T>(t) / std​::​forward<U>(u)[.](#arithmetic.operations.divides-2.sentence-1)

#### [22.10.7.6](#arithmetic.operations.modulus) Class template modulus [[arithmetic.operations.modulus]](arithmetic.operations.modulus)

[🔗](#lib:modulus)

`template<class T = void> struct modulus {
  constexpr T operator()(const T& x, const T& y) const;
};
`

[🔗](#lib:operator(),modulus)

`constexpr T operator()(const T& x, const T& y) const;
`

[1](#arithmetic.operations.modulus-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12186)

*Returns*: x % y[.](#arithmetic.operations.modulus-1.sentence-1)

[🔗](#lib:modulus%3c%3e)

`template<> struct modulus<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) % std::forward<U>(u));

  using is_transparent = unspecified;
};
`

[🔗](#lib:operator(),modulus%3c%3e)

`template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) % std::forward<U>(u));
`

[2](#arithmetic.operations.modulus-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12208)

*Returns*: std​::​forward<T>(t) % std​::​forward<U>(u)[.](#arithmetic.operations.modulus-2.sentence-1)

#### [22.10.7.7](#arithmetic.operations.negate) Class template negate [[arithmetic.operations.negate]](arithmetic.operations.negate)

[🔗](#lib:negate)

`template<class T = void> struct negate {
  constexpr T operator()(const T& x) const;
};
`

[🔗](#lib:operator(),negate)

`constexpr T operator()(const T& x) const;
`

[1](#arithmetic.operations.negate-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12228)

*Returns*: -x[.](#arithmetic.operations.negate-1.sentence-1)

[🔗](#lib:negate%3c%3e)

`template<> struct negate<void> {
  template<class T> constexpr auto operator()(T&& t) const
    -> decltype(-std::forward<T>(t));

  using is_transparent = unspecified;
};
`

[🔗](#lib:operator(),negate%3c%3e)

`template<class T> constexpr auto operator()(T&& t) const
    -> decltype(-std::forward<T>(t));
`

[2](#arithmetic.operations.negate-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12250)

*Returns*: -std​::​forward<T>(t)[.](#arithmetic.operations.negate-2.sentence-1)

### [22.10.8](#comparisons) Comparisons [[comparisons]](comparisons)

#### [22.10.8.1](#comparisons.general) General [[comparisons.general]](comparisons.general)

[1](#comparisons.general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12260)

The library provides basic function object classes for all of the comparison
operators in the language ([[expr.rel]](expr.rel "7.6.9 Relational operators"), [[expr.eq]](expr.eq "7.6.10 Equality operators"))[.](#comparisons.general-1.sentence-1)

[2](#comparisons.general-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12264)

For templates less, greater, less_equal, andgreater_equal, the specializations for any pointer type
yield a result consistent with the
implementation-defined strict total order over pointers ([[defns.order.ptr]](defns.order.ptr "3.28 implementation-defined strict total order over pointers"))[.](#comparisons.general-2.sentence-1)

[*Note [1](#comparisons.general-note-1)*:

If a < b is well-defined
for pointers a and b of type P,
then (a < b) == less<P>()(a, b),(a > b) == greater<P>()(a, b), and so forth[.](#comparisons.general-2.sentence-2)

— *end note*]

For template specializations less<void>, greater<void>,less_equal<void>, and greater_equal<void>,
if the call operator calls a built-in operator comparing pointers,
the call operator yields a result consistent
with the implementation-defined strict total order over pointers[.](#comparisons.general-2.sentence-3)

#### [22.10.8.2](#comparisons.equal.to) Class template equal_to [[comparisons.equal.to]](comparisons.equal.to)

[🔗](#lib:equal_to)

`template<class T = void> struct equal_to {
  constexpr bool operator()(const T& x, const T& y) const;
};
`

[🔗](#lib:operator(),equal_to)

`constexpr bool operator()(const T& x, const T& y) const;
`

[1](#comparisons.equal.to-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12296)

*Returns*: x == y[.](#comparisons.equal.to-1.sentence-1)

[🔗](#lib:equal_to%3c%3e)

`template<> struct equal_to<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) == std::forward<U>(u));

  using is_transparent = unspecified;
};
`

[🔗](#lib:operator(),equal_to%3c%3e)

`template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) == std::forward<U>(u));
`

[2](#comparisons.equal.to-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12318)

*Returns*: std​::​forward<T>(t) == std​::​forward<U>(u)[.](#comparisons.equal.to-2.sentence-1)

#### [22.10.8.3](#comparisons.not.equal.to) Class template not_equal_to [[comparisons.not.equal.to]](comparisons.not.equal.to)

[🔗](#lib:not_equal_to)

`template<class T = void> struct not_equal_to {
  constexpr bool operator()(const T& x, const T& y) const;
};
`

[🔗](#lib:operator(),not_equal_to)

`constexpr bool operator()(const T& x, const T& y) const;
`

[1](#comparisons.not.equal.to-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12338)

*Returns*: x != y[.](#comparisons.not.equal.to-1.sentence-1)

[🔗](#lib:not_equal_to%3c%3e)

`template<> struct not_equal_to<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) != std::forward<U>(u));

  using is_transparent = unspecified;
};
`

[🔗](#lib:operator(),not_equal_to%3c%3e)

`template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) != std::forward<U>(u));
`

[2](#comparisons.not.equal.to-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12360)

*Returns*: std​::​forward<T>(t) != std​::​forward<U>(u)[.](#comparisons.not.equal.to-2.sentence-1)

#### [22.10.8.4](#comparisons.greater) Class template greater [[comparisons.greater]](comparisons.greater)

[🔗](#lib:greater)

`template<class T = void> struct greater {
  constexpr bool operator()(const T& x, const T& y) const;
};
`

[🔗](#lib:operator(),greater)

`constexpr bool operator()(const T& x, const T& y) const;
`

[1](#comparisons.greater-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12380)

*Returns*: x > y[.](#comparisons.greater-1.sentence-1)

[🔗](#lib:greater%3c%3e)

`template<> struct greater<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) > std::forward<U>(u));

  using is_transparent = unspecified;
};
`

[🔗](#lib:operator(),greater%3c%3e)

`template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) > std::forward<U>(u));
`

[2](#comparisons.greater-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12402)

*Returns*: std​::​forward<T>(t) > std​::​forward<U>(u)[.](#comparisons.greater-2.sentence-1)

#### [22.10.8.5](#comparisons.less) Class template less [[comparisons.less]](comparisons.less)

[🔗](#lib:less)

`template<class T = void> struct less {
  constexpr bool operator()(const T& x, const T& y) const;
};
`

[🔗](#lib:operator(),less)

`constexpr bool operator()(const T& x, const T& y) const;
`

[1](#comparisons.less-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12422)

*Returns*: x < y[.](#comparisons.less-1.sentence-1)

[🔗](#lib:less%3c%3e)

`template<> struct less<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) < std::forward<U>(u));

  using is_transparent = unspecified;
};
`

[🔗](#lib:operator(),less%3c%3e)

`template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) < std::forward<U>(u));
`

[2](#comparisons.less-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12444)

*Returns*: std​::​forward<T>(t) < std​::​forward<U>(u)[.](#comparisons.less-2.sentence-1)

#### [22.10.8.6](#comparisons.greater.equal) Class template greater_equal [[comparisons.greater.equal]](comparisons.greater.equal)

[🔗](#lib:greater_equal)

`template<class T = void> struct greater_equal {
  constexpr bool operator()(const T& x, const T& y) const;
};
`

[🔗](#lib:operator(),greater_equal)

`constexpr bool operator()(const T& x, const T& y) const;
`

[1](#comparisons.greater.equal-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12464)

*Returns*: x >= y[.](#comparisons.greater.equal-1.sentence-1)

[🔗](#lib:greater_equal%3c%3e)

`template<> struct greater_equal<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) >= std::forward<U>(u));

  using is_transparent = unspecified;
};
`

[🔗](#lib:operator(),greater_equal%3c%3e)

`template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) >= std::forward<U>(u));
`

[2](#comparisons.greater.equal-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12486)

*Returns*: std​::​forward<T>(t) >= std​::​forward<U>(u)[.](#comparisons.greater.equal-2.sentence-1)

#### [22.10.8.7](#comparisons.less.equal) Class template less_equal [[comparisons.less.equal]](comparisons.less.equal)

[🔗](#lib:less_equal)

`template<class T = void> struct less_equal {
  constexpr bool operator()(const T& x, const T& y) const;
};
`

[🔗](#lib:operator(),less_equal)

`constexpr bool operator()(const T& x, const T& y) const;
`

[1](#comparisons.less.equal-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12506)

*Returns*: x <= y[.](#comparisons.less.equal-1.sentence-1)

[🔗](#lib:less_equal%3c%3e)

`template<> struct less_equal<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) <= std::forward<U>(u));

  using is_transparent = unspecified;
};
`

[🔗](#lib:operator(),less_equal%3c%3e)

`template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) <= std::forward<U>(u));
`

[2](#comparisons.less.equal-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12528)

*Returns*: std​::​forward<T>(t) <= std​::​forward<U>(u)[.](#comparisons.less.equal-2.sentence-1)

#### [22.10.8.8](#comparisons.three.way) Class compare_three_way [[comparisons.three.way]](comparisons.three.way)

[🔗](#lib:compare_three_way)

namespace std {struct compare_three_way {template<class T, class U>constexpr auto operator()(T&& t, U&& u) const; using is_transparent = *unspecified*; };}

[🔗](#comparisons.three.way-itemdecl:1)

`template<class T, class U>
  constexpr auto operator()(T&& t, U&& u) const;
`

[1](#comparisons.three.way-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12553)

*Constraints*: T and U satisfy [three_way_comparable_with](cmp.concept#concept:three_way_comparable_with "17.12.4 Concept three_­way_­comparable [cmp.concept]")[.](#comparisons.three.way-1.sentence-1)

[2](#comparisons.three.way-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12557)

*Preconditions*: If the expression std​::​forward<T>(t) <=> std​::​forward<U>(u) results in
a call to a built-in operator <=> comparing pointers of type P,
the conversion sequences from both T and U to P are equality-preserving ([[concepts.equality]](concepts.equality "18.2 Equality preservation"));
otherwise, T and U model [three_way_comparable_with](cmp.concept#concept:three_way_comparable_with "17.12.4 Concept three_­way_­comparable [cmp.concept]")[.](#comparisons.three.way-2.sentence-1)

[3](#comparisons.three.way-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12565)

*Effects*: 

- [(3.1)](#comparisons.three.way-3.1)

If the expression std​::​forward<T>(t) <=> std​::​forward<U>(u) results in
 a call to a built-in operator <=> comparing pointers of type P,
 returns strong_ordering​::​less if (the converted value of) t precedes u in the implementation-defined strict total order
 over pointers ([[defns.order.ptr]](defns.order.ptr "3.28 implementation-defined strict total order over pointers")), strong_ordering​::​greater if u precedes t, and
 otherwise strong_ordering​::​equal.

- [(3.2)](#comparisons.three.way-3.2)

Otherwise, equivalent to: return std​::​forward<T>(t) <=> std​::​forward<U>(u);

### [22.10.9](#range.cmp) Concept-constrained comparisons [[range.cmp]](range.cmp)

[🔗](#lib:equal_to_)

struct ranges::equal_to {template<class T, class U>constexpr bool operator()(T&& t, U&& u) const; using is_transparent = *unspecified*;};

[🔗](#range.cmp-itemdecl:1)

`template<class T, class U>
  constexpr bool operator()(T&& t, U&& u) const;
`

[1](#range.cmp-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12601)

*Constraints*: T and U satisfy [equality_comparable_with](concept.equalitycomparable#concept:equality_comparable_with "18.5.4 Concept equality_­comparable [concept.equalitycomparable]")[.](#range.cmp-1.sentence-1)

[2](#range.cmp-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12605)

*Preconditions*: If the expression std​::​forward<T>(t) == std​::​forward<U>(u) results in a call to a built-in operator == comparing pointers of typeP, the conversion sequences from both T and U to P are equality-preserving ([[concepts.equality]](concepts.equality "18.2 Equality preservation"));
otherwise, T and U model [equality_comparable_with](concept.equalitycomparable#concept:equality_comparable_with "18.5.4 Concept equality_­comparable [concept.equalitycomparable]")[.](#range.cmp-2.sentence-1)

[3](#range.cmp-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12613)

*Effects*: 

- [(3.1)](#range.cmp-3.1)

If the expression std​::​forward<T>(t) == std​::​forward<U>(u) results in
 a call to a built-in operator == comparing pointers:
 returns false if either (the converted value of) t precedes u or u precedes t in the implementation-defined strict
 total order over pointers ([[defns.order.ptr]](defns.order.ptr "3.28 implementation-defined strict total order over pointers")) and otherwise true.

- [(3.2)](#range.cmp-3.2)

Otherwise, equivalent to: return std​::​forward<T>(t) == std​::​forward<U>(u);

[🔗](#lib:not_equal_to_)

struct ranges::not_equal_to {template<class T, class U>constexpr bool operator()(T&& t, U&& u) const; using is_transparent = *unspecified*;};

[🔗](#range.cmp-itemdecl:2)

`template<class T, class U>
  constexpr bool operator()(T&& t, U&& u) const;
`

[4](#range.cmp-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12645)

*Constraints*: T and U satisfy [equality_comparable_with](concept.equalitycomparable#concept:equality_comparable_with "18.5.4 Concept equality_­comparable [concept.equalitycomparable]")[.](#range.cmp-4.sentence-1)

[5](#range.cmp-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12649)

*Effects*: Equivalent to:return !ranges::equal_to{}(std::forward<T>(t), std::forward<U>(u));

[🔗](#lib:greater_)

struct ranges::greater {template<class T, class U>constexpr bool operator()(T&& t, U&& u) const; using is_transparent = *unspecified*;};

[🔗](#range.cmp-itemdecl:3)

`template<class T, class U>
  constexpr bool operator()(T&& t, U&& u) const;
`

[6](#range.cmp-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12673)

*Constraints*: T and U satisfy [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")[.](#range.cmp-6.sentence-1)

[7](#range.cmp-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12677)

*Effects*: Equivalent to:return ranges::less{}(std::forward<U>(u), std::forward<T>(t));

[🔗](#lib:less_)

struct ranges::less {template<class T, class U>constexpr bool operator()(T&& t, U&& u) const; using is_transparent = *unspecified*;};

[🔗](#range.cmp-itemdecl:4)

`template<class T, class U>
  constexpr bool operator()(T&& t, U&& u) const;
`

[8](#range.cmp-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12701)

*Constraints*: T and U satisfy [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")[.](#range.cmp-8.sentence-1)

[9](#range.cmp-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12705)

*Preconditions*: If the expression std​::​forward<T>(t) < std​::​forward<U>(u) results in a
call to a built-in operator < comparing pointers of type P, the
conversion sequences from both T and U to P are
equality-preserving ([[concepts.equality]](concepts.equality "18.2 Equality preservation"));
otherwise, T and U model [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")[.](#range.cmp-9.sentence-1)

For any expressionsET and EU such that decltype((ET)) is T anddecltype((EU)) is U, exactly one ofranges​::​less{}(ET, EU),ranges​::​less{}(EU, ET), orranges​::​equal_to{}(ET, EU) is true[.](#range.cmp-9.sentence-2)

[10](#range.cmp-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12720)

*Effects*: 

- [(10.1)](#range.cmp-10.1)

If the expression std​::​forward<T>(t) < std​::​forward<U>(u) results in a
call to a built-in operator < comparing pointers:
returns true if (the converted value of) t precedes u in
the implementation-defined strict total order over pointers ([[defns.order.ptr]](defns.order.ptr "3.28 implementation-defined strict total order over pointers"))
and otherwise false.

- [(10.2)](#range.cmp-10.2)

Otherwise, equivalent to:return std​::​forward<T>(t) < std​::​forward<U>(u);

[🔗](#lib:greater_equal_)

struct ranges::greater_equal {template<class T, class U>constexpr bool operator()(T&& t, U&& u) const; using is_transparent = *unspecified*;};

[🔗](#range.cmp-itemdecl:5)

`template<class T, class U>
  constexpr bool operator()(T&& t, U&& u) const;
`

[11](#range.cmp-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12752)

*Constraints*: T and U satisfy [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")[.](#range.cmp-11.sentence-1)

[12](#range.cmp-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12756)

*Effects*: Equivalent to:return !ranges::less{}(std::forward<T>(t), std::forward<U>(u));

[🔗](#lib:less_equal_)

`struct ranges::less_equal {
  template<class T, class U>
    constexpr bool operator()(T&& t, U&& u) const;

  using is_transparent = unspecified;
};
`

[🔗](#range.cmp-itemdecl:7)

`template<class T, class U>
  constexpr bool operator()(T&& t, U&& u) const;
`

[13](#range.cmp-13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12780)

*Constraints*: T and U satisfy [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")[.](#range.cmp-13.sentence-1)

[14](#range.cmp-14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12784)

*Effects*: Equivalent to:return !ranges::less{}(std::forward<U>(u), std::forward<T>(t));

### [22.10.10](#logical.operations) Logical operations [[logical.operations]](logical.operations)

#### [22.10.10.1](#logical.operations.general) General [[logical.operations.general]](logical.operations.general)

[1](#logical.operations.general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12796)

The library provides basic function object classes for all of the logical
operators in the language ([[expr.log.and]](expr.log.and "7.6.14 Logical AND operator"), [[expr.log.or]](expr.log.or "7.6.15 Logical OR operator"), [[expr.unary.op]](expr.unary.op "7.6.2.2 Unary operators"))[.](#logical.operations.general-1.sentence-1)

#### [22.10.10.2](#logical.operations.and) Class template logical_and [[logical.operations.and]](logical.operations.and)

[🔗](#lib:logical_and)

`template<class T = void> struct logical_and {
  constexpr bool operator()(const T& x, const T& y) const;
};
`

[🔗](#lib:operator(),logical_and)

`constexpr bool operator()(const T& x, const T& y) const;
`

[1](#logical.operations.and-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12815)

*Returns*: x && y[.](#logical.operations.and-1.sentence-1)

[🔗](#lib:logical_and%3c%3e)

`template<> struct logical_and<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) && std::forward<U>(u));

  using is_transparent = unspecified;
};
`

[🔗](#lib:operator(),logical_and%3c%3e)

`template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) && std::forward<U>(u));
`

[2](#logical.operations.and-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12837)

*Returns*: std​::​forward<T>(t) && std​::​forward<U>(u)[.](#logical.operations.and-2.sentence-1)

#### [22.10.10.3](#logical.operations.or) Class template logical_or [[logical.operations.or]](logical.operations.or)

[🔗](#lib:logical_or)

`template<class T = void> struct logical_or {
  constexpr bool operator()(const T& x, const T& y) const;
};
`

[🔗](#lib:operator(),logical_or)

`constexpr bool operator()(const T& x, const T& y) const;
`

[1](#logical.operations.or-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12857)

*Returns*: x || y[.](#logical.operations.or-1.sentence-1)

[🔗](#lib:logical_or%3c%3e)

`template<> struct logical_or<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) || std::forward<U>(u));

  using is_transparent = unspecified;
};
`

[🔗](#lib:operator(),logical_or%3c%3e)

`template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) || std::forward<U>(u));
`

[2](#logical.operations.or-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12879)

*Returns*: std​::​forward<T>(t) || std​::​forward<U>(u)[.](#logical.operations.or-2.sentence-1)

#### [22.10.10.4](#logical.operations.not) Class template logical_not [[logical.operations.not]](logical.operations.not)

[🔗](#lib:logical_not)

`template<class T = void> struct logical_not {
  constexpr bool operator()(const T& x) const;
};
`

[🔗](#lib:operator(),logical_not)

`constexpr bool operator()(const T& x) const;
`

[1](#logical.operations.not-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12899)

*Returns*: !x[.](#logical.operations.not-1.sentence-1)

[🔗](#lib:logical_not%3c%3e)

`template<> struct logical_not<void> {
  template<class T> constexpr auto operator()(T&& t) const
    -> decltype(!std::forward<T>(t));

  using is_transparent = unspecified;
};
`

[🔗](#lib:operator(),logical_not%3c%3e)

`template<class T> constexpr auto operator()(T&& t) const
    -> decltype(!std::forward<T>(t));
`

[2](#logical.operations.not-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12921)

*Returns*: !std​::​forward<T>(t)[.](#logical.operations.not-2.sentence-1)

### [22.10.11](#bitwise.operations) Bitwise operations [[bitwise.operations]](bitwise.operations)

#### [22.10.11.1](#bitwise.operations.general) General [[bitwise.operations.general]](bitwise.operations.general)

[1](#bitwise.operations.general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12931)

The library provides basic function object classes for all of the bitwise
operators in the language ([[expr.bit.and]](expr.bit.and "7.6.11 Bitwise AND operator"), [[expr.or]](expr.or "7.6.13 Bitwise inclusive OR operator"), [[expr.xor]](expr.xor "7.6.12 Bitwise exclusive OR operator"), [[expr.unary.op]](expr.unary.op "7.6.2.2 Unary operators"))[.](#bitwise.operations.general-1.sentence-1)

#### [22.10.11.2](#bitwise.operations.and) Class template bit_and [[bitwise.operations.and]](bitwise.operations.and)

[🔗](#lib:bit_and)

`template<class T = void> struct bit_and {
  constexpr T operator()(const T& x, const T& y) const;
};
`

[🔗](#lib:operator(),bit_and)

`constexpr T operator()(const T& x, const T& y) const;
`

[1](#bitwise.operations.and-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12950)

*Returns*: x & y[.](#bitwise.operations.and-1.sentence-1)

[🔗](#lib:bit_and%3c%3e)

`template<> struct bit_and<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) & std::forward<U>(u));

  using is_transparent = unspecified;
};
`

[🔗](#lib:operator(),bit_and%3c%3e)

`template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) & std::forward<U>(u));
`

[2](#bitwise.operations.and-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12972)

*Returns*: std​::​forward<T>(t) & std​::​forward<U>(u)[.](#bitwise.operations.and-2.sentence-1)

#### [22.10.11.3](#bitwise.operations.or) Class template bit_or [[bitwise.operations.or]](bitwise.operations.or)

[🔗](#lib:bit_or)

`template<class T = void> struct bit_or {
  constexpr T operator()(const T& x, const T& y) const;
};
`

[🔗](#lib:operator(),bit_or)

`constexpr T operator()(const T& x, const T& y) const;
`

[1](#bitwise.operations.or-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L12992)

*Returns*: x | y[.](#bitwise.operations.or-1.sentence-1)

[🔗](#lib:bit_or%3c%3e)

`template<> struct bit_or<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) | std::forward<U>(u));

  using is_transparent = unspecified;
};
`

[🔗](#lib:operator(),bit_or%3c%3e)

`template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) | std::forward<U>(u));
`

[2](#bitwise.operations.or-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13014)

*Returns*: std​::​forward<T>(t) | std​::​forward<U>(u)[.](#bitwise.operations.or-2.sentence-1)

#### [22.10.11.4](#bitwise.operations.xor) Class template bit_xor [[bitwise.operations.xor]](bitwise.operations.xor)

[🔗](#lib:bit_xor)

`template<class T = void> struct bit_xor {
  constexpr T operator()(const T& x, const T& y) const;
};
`

[🔗](#lib:operator(),bit_xor)

`constexpr T operator()(const T& x, const T& y) const;
`

[1](#bitwise.operations.xor-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13034)

*Returns*: x ^ y[.](#bitwise.operations.xor-1.sentence-1)

[🔗](#lib:bit_xor%3c%3e)

`template<> struct bit_xor<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) ^ std::forward<U>(u));

  using is_transparent = unspecified;
};
`

[🔗](#lib:operator(),bit_xor%3c%3e)

`template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) ^ std::forward<U>(u));
`

[2](#bitwise.operations.xor-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13056)

*Returns*: std​::​forward<T>(t) ^ std​::​forward<U>(u)[.](#bitwise.operations.xor-2.sentence-1)

#### [22.10.11.5](#bitwise.operations.not) Class template bit_not [[bitwise.operations.not]](bitwise.operations.not)

[🔗](#bitwise.operations.not-itemdecl:1)

`template<class T = void> struct bit_not {
  constexpr T operator()(const T& x) const;
};
`

[🔗](#lib:operator(),bit_not)

`constexpr T operator()(const T& x) const;
`

[1](#bitwise.operations.not-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13075)

*Returns*: ~x[.](#bitwise.operations.not-1.sentence-1)

[🔗](#lib:bit_not%3c%3e)

`template<> struct bit_not<void> {
  template<class T> constexpr auto operator()(T&& t) const
    -> decltype(~std::forward<T>(t));

  using is_transparent = unspecified;
};
`

[🔗](#lib:operator(),bit_not%3c%3e)

`template<class T> constexpr auto operator()(T&& t) const
    -> decltype(~std::forward<T>(t));
`

[2](#bitwise.operations.not-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13097)

*Returns*: ~std​::​forward<T>(t)[.](#bitwise.operations.not-2.sentence-1)

### [22.10.12](#func.identity) Class identity [[func.identity]](func.identity)

[🔗](#lib:identity)

`struct identity {
  template<class T>
    constexpr T&& operator()(T&& t) const noexcept;

  using is_transparent = unspecified;
};

template<class T>
  constexpr T&& operator()(T&& t) const noexcept;
`

[1](#func.identity-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13119)

*Effects*: Equivalent to: return std​::​forward<T>(t);

### [22.10.13](#func.not.fn) Function template not_fn [[func.not.fn]](func.not.fn)

[🔗](#lib:not_fn)

`template<class F> constexpr unspecified not_fn(F&& f);
`

[1](#func.not.fn-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13133)

In the text that follows:

- [(1.1)](#func.not.fn-1.1)

g is a value of the result of a not_fn invocation,

- [(1.2)](#func.not.fn-1.2)

FD is the type decay_t<F>,

- [(1.3)](#func.not.fn-1.3)

fd is the target object of g ([[func.def]](#func.def "22.10.3 Definitions"))
 of type FD,
 direct-non-list-initialized with std​::​forward<F​>(f),

- [(1.4)](#func.not.fn-1.4)

call_args is an argument pack
 used in a function call expression ([[expr.call]](expr.call "7.6.1.3 Function call")) of g[.](#func.not.fn-1.sentence-1)

[2](#func.not.fn-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13145)

*Mandates*: is_constructible_v<FD, F> && is_move_constructible_v<FD> is true[.](#func.not.fn-2.sentence-1)

[3](#func.not.fn-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13150)

*Preconditions*: FD meets the [*Cpp17MoveConstructible*](utility.arg.requirements#:Cpp17MoveConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements[.](#func.not.fn-3.sentence-1)

[4](#func.not.fn-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13154)

*Returns*: A perfect forwarding call wrapper ([[func.require]](#func.require#term.perfect.forwarding.call.wrapper "22.10.4 Requirements")) g with call pattern !invoke(fd, call_args...)[.](#func.not.fn-4.sentence-1)

[5](#func.not.fn-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13159)

*Throws*: Any exception thrown by the initialization of fd[.](#func.not.fn-5.sentence-1)

[🔗](#lib:not_fn_)

`template<auto f> constexpr unspecified not_fn() noexcept;
`

[6](#func.not.fn-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13170)

In the text that follows:

- [(6.1)](#func.not.fn-6.1)

F is the type of f,

- [(6.2)](#func.not.fn-6.2)

g is a value of the result of a not_fn invocation,

- [(6.3)](#func.not.fn-6.3)

call_args is an argument pack
used in a function call expression ([[expr.call]](expr.call "7.6.1.3 Function call")) of g[.](#func.not.fn-6.sentence-1)

[7](#func.not.fn-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13182)

*Mandates*: If is_pointer_v<F> || is_member_pointer_v<F> is true,
then f != nullptr is true[.](#func.not.fn-7.sentence-1)

[8](#func.not.fn-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13187)

*Returns*: A perfect forwarding call wrapper ([[func.require]](#func.require "22.10.4 Requirements")) g that
does not have state entities, and
has the call pattern !invoke(f, call_args...)[.](#func.not.fn-8.sentence-1)

### [22.10.14](#func.bind.partial) Function templates bind_front and bind_back [[func.bind.partial]](func.bind.partial)

[🔗](#lib:bind_front)

`template<class F, class... Args>
  constexpr unspecified bind_front(F&& f, Args&&... args);
template<class F, class... Args>
  constexpr unspecified bind_back(F&& f, Args&&... args);
`

[1](#func.bind.partial-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13206)

Within this subclause:

- [(1.1)](#func.bind.partial-1.1)

g is a value of
the result of a bind_front or bind_back invocation,

- [(1.2)](#func.bind.partial-1.2)

FD is the type decay_t<F>,

- [(1.3)](#func.bind.partial-1.3)

fd is the target object of g ([[func.def]](#func.def "22.10.3 Definitions"))
 of type FD,
 direct-non-list-initialized with std​::​forward<F​>(f),

- [(1.4)](#func.bind.partial-1.4)

BoundArgs is a pack
 that denotes decay_t<Args>...,

- [(1.5)](#func.bind.partial-1.5)

bound_args is
 a pack of bound argument entities of g ([[func.def]](#func.def "22.10.3 Definitions"))
 of types BoundArgs...,
 direct-non-list-initialized with std​::​forward<Args>(args)...,
 respectively, and

- [(1.6)](#func.bind.partial-1.6)

call_args is an argument pack used in
 a function call expression ([[expr.call]](expr.call "7.6.1.3 Function call")) of g[.](#func.bind.partial-1.sentence-1)

[2](#func.bind.partial-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13226)

*Mandates*: is_constructible_v<FD, F> && is_move_constructible_v<FD> &&(is_constructible_v<BoundArgs, Args> && ...) &&(is_move_constructible_v<BoundArgs> && ...) is true[.](#func.bind.partial-2.sentence-1)

[3](#func.bind.partial-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13236)

*Preconditions*: FD meets the [*Cpp17MoveConstructible*](utility.arg.requirements#:Cpp17MoveConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements[.](#func.bind.partial-3.sentence-1)

For each Ti in BoundArgs,
if Ti is an object type,Ti meets the [*Cpp17MoveConstructible*](utility.arg.requirements#:Cpp17MoveConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements[.](#func.bind.partial-3.sentence-2)

[4](#func.bind.partial-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13243)

*Returns*: A perfect forwarding call wrapper ([[func.require]](#func.require#term.perfect.forwarding.call.wrapper "22.10.4 Requirements")) g with call pattern:

- [(4.1)](#func.bind.partial-4.1)

invoke(fd, bound_args..., call_args...) for a bind_front invocation, or

- [(4.2)](#func.bind.partial-4.2)

invoke(fd, call_args..., bound_args...) for a bind_back invocation[.](#func.bind.partial-4.sentence-1)

[5](#func.bind.partial-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13256)

*Throws*: Any exception thrown by
the initialization of the state entities of g ([[func.def]](#func.def "22.10.3 Definitions"))[.](#func.bind.partial-5.sentence-1)

[🔗](#lib:bind_front_)

`template<auto f, class... Args>
  constexpr unspecified bind_front(Args&&... args);
template<auto f, class... Args>
  constexpr unspecified bind_back(Args&&... args);
`

[6](#func.bind.partial-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13272)

Within this subclause:

- [(6.1)](#func.bind.partial-6.1)

F is the type of f,

- [(6.2)](#func.bind.partial-6.2)

g is a value of the result of
a bind_front or bind_back invocation,

- [(6.3)](#func.bind.partial-6.3)

BoundArgs is a pack that denotes decay_t<Args>...,

- [(6.4)](#func.bind.partial-6.4)

bound_args is a pack of bound argument entities ofg ([[func.def]](#func.def "22.10.3 Definitions")) of types BoundArgs...,
direct-non-list-initialized with std​::​forward<Args>(args)...,
respectively, and

- [(6.5)](#func.bind.partial-6.5)

call_args is an argument pack used in
a function call expression ([[expr.call]](expr.call "7.6.1.3 Function call")) of g[.](#func.bind.partial-6.sentence-1)

[7](#func.bind.partial-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13292)

*Mandates*: 

- [(7.1)](#func.bind.partial-7.1)

(is_constructible_v<BoundArgs, Args> && ...) is true, and

- [(7.2)](#func.bind.partial-7.2)

(is_move_constructible_v<BoundArgs> && ...) is true, and

- [(7.3)](#func.bind.partial-7.3)

if is_pointer_v<F> || is_member_pointer_v<F> is true,
then f != nullptr is true[.](#func.bind.partial-7.sentence-1)

[8](#func.bind.partial-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13304)

*Preconditions*: For each Ti in BoundArgs,Ti meets the [*Cpp17MoveConstructible*](utility.arg.requirements#:Cpp17MoveConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements[.](#func.bind.partial-8.sentence-1)

[9](#func.bind.partial-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13309)

*Returns*: A perfect forwarding call wrapper ([[func.require]](#func.require "22.10.4 Requirements")) g that
does not have a target object, and has the call pattern:

- [(9.1)](#func.bind.partial-9.1)

invoke(f, bound_args..., call_args...) for a bind_front invocation, or

- [(9.2)](#func.bind.partial-9.2)

invoke(f, call_args..., bound_args...) for a bind_back invocation[.](#func.bind.partial-9.sentence-1)

[10](#func.bind.partial-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13322)

*Throws*: Any exception thrown by the initialization of bound_args[.](#func.bind.partial-10.sentence-1)

### [22.10.15](#func.bind) Function object binders [[func.bind]](func.bind)

#### [22.10.15.1](#func.bind.general) General [[func.bind.general]](func.bind.general)

[1](#func.bind.general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13332)

Subclause [[func.bind]](#func.bind "22.10.15 Function object binders") describes a uniform mechanism for binding
arguments of callable objects[.](#func.bind.general-1.sentence-1)

#### [22.10.15.2](#func.bind.isbind) Class template is_bind_expression [[func.bind.isbind]](func.bind.isbind)

[🔗](#lib:is_bind_expression)

namespace std {template<class T> struct is_bind_expression; // see below}

[1](#func.bind.isbind-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13345)

The class template is_bind_expression can be used to detect function objects
generated by bind[.](#func.bind.isbind-1.sentence-1)

The function template bind uses is_bind_expression to detect subexpressions[.](#func.bind.isbind-1.sentence-2)

[2](#func.bind.isbind-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13350)

Specializations of the is_bind_expression template shall meet
the [*Cpp17UnaryTypeTrait*](meta.rqmts#:Cpp17UnaryTypeTrait "21.3.2 Requirements [meta.rqmts]") requirements ([[meta.rqmts]](meta.rqmts "21.3.2 Requirements"))[.](#func.bind.isbind-2.sentence-1)

The implementation
provides a definition that has a base characteristic oftrue_type if T is a type returned from bind,
otherwise it has a base characteristic of false_type[.](#func.bind.isbind-2.sentence-2)

A program may specialize this template for a program-defined type T to have a base characteristic of true_type to indicate thatT should be treated as a subexpression in a bind call[.](#func.bind.isbind-2.sentence-3)

#### [22.10.15.3](#func.bind.isplace) Class template is_placeholder [[func.bind.isplace]](func.bind.isplace)

[🔗](#lib:is_placeholder)

namespace std {template<class T> struct is_placeholder; // see below}

[1](#func.bind.isplace-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13369)

The class template is_placeholder can be used to detect the standard placeholders_1, _2, and so on ([[func.bind.place]](#func.bind.place "22.10.15.5 Placeholders"))[.](#func.bind.isplace-1.sentence-1)

The function template bind usesis_placeholder to detect placeholders[.](#func.bind.isplace-1.sentence-2)

[2](#func.bind.isplace-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13386)

Specializations of the is_placeholder template shall meet
the [*Cpp17UnaryTypeTrait*](meta.rqmts#:Cpp17UnaryTypeTrait "21.3.2 Requirements [meta.rqmts]") requirements ([[meta.rqmts]](meta.rqmts "21.3.2 Requirements"))[.](#func.bind.isplace-2.sentence-1)

The implementation
provides a definition that has the base characteristic ofintegral_constant<int, *J*> if T is the type ofstd​::​placeholders​::​_*J*, otherwise it has a
base characteristic of integral_constant<int, 0>[.](#func.bind.isplace-2.sentence-2)

A program
may specialize this template for a program-defined type T to
have a base characteristic of integral_constant<int, N> with N > 0 to indicate that T should be
treated as a placeholder type[.](#func.bind.isplace-2.sentence-3)

#### [22.10.15.4](#func.bind.bind) Function template bind [[func.bind.bind]](func.bind.bind)

[1](#func.bind.bind-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13401)

In the text that follows:

- [(1.1)](#func.bind.bind-1.1)

g is a value of the result of a bind invocation,

- [(1.2)](#func.bind.bind-1.2)

FD is the type decay_t<F>,

- [(1.3)](#func.bind.bind-1.3)

fd is an lvalue that
 is a target object of g ([[func.def]](#func.def "22.10.3 Definitions")) of type FD direct-non-list-initialized with std​::​forward<F>(f),

- [(1.4)](#func.bind.bind-1.4)

Ti is the ith type in the template parameter pack BoundArgs,

- [(1.5)](#func.bind.bind-1.5)

TDi is the type decay_t<Ti>,

- [(1.6)](#func.bind.bind-1.6)

ti is the ith argument in the function parameter pack bound_args,

- [(1.7)](#func.bind.bind-1.7)

tdi is a bound argument entity
 of g ([[func.def]](#func.def "22.10.3 Definitions")) of type TDi direct-non-list-initialized with std​::​forward<​Ti>(ti),

- [(1.8)](#func.bind.bind-1.8)

Uj is the jth deduced type of the UnBoundArgs&&... parameter
 of the argument forwarding call wrapper, and

- [(1.9)](#func.bind.bind-1.9)

uj is the jth argument associated with Uj[.](#func.bind.bind-1.sentence-1)

[🔗](#lib:bind_)

`template<class F, class... BoundArgs>
  constexpr unspecified bind(F&& f, BoundArgs&&... bound_args);
template<class R, class F, class... BoundArgs>
  constexpr unspecified bind(F&& f, BoundArgs&&... bound_args);
`

[2](#func.bind.bind-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13430)

*Mandates*: is_constructible_v<FD, F> is true[.](#func.bind.bind-2.sentence-1)

For each Ti in BoundArgs, is_constructible_v<TDi, Ti> is true[.](#func.bind.bind-2.sentence-2)

[3](#func.bind.bind-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13435)

*Preconditions*: FD and each TDi meet
the [*Cpp17MoveConstructible*](utility.arg.requirements#:Cpp17MoveConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") and [*Cpp17Destructible*](utility.arg.requirements#:Cpp17Destructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements[.](#func.bind.bind-3.sentence-1)

*INVOKE*(fd, w1, w2, …,wN) ([[func.require]](#func.require "22.10.4 Requirements")) is a valid expression for some
values w1, w2, …, wN, whereN has the value sizeof...(bound_args)[.](#func.bind.bind-3.sentence-2)

[4](#func.bind.bind-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13444)

*Returns*: An argument forwarding call wrapper g ([[func.require]](#func.require "22.10.4 Requirements"))[.](#func.bind.bind-4.sentence-1)

A program that attempts to invoke a volatile-qualified g is ill-formed[.](#func.bind.bind-4.sentence-2)

When g is not volatile-qualified, invocation ofg(u1, u2, …, uM) is expression-equivalent ([[defns.expression.equivalent]](defns.expression.equivalent "3.22 expression-equivalent")) to*INVOKE*(static_cast<Vfd>(vfd), static_cast<V1>(v1), static_cast<V2>(v2), …, static_cast<VN>(vN)) for the first overload, and*INVOKE*<R>(static_cast<Vfd>(vfd), static_cast<V1>(v1), static_cast<V2>(v2), …, static_cast<VN>(vN)) for the second overload,
where the values and types of the target argument vfd and
of the bound argumentsv1, v2, …, vN are determined as specified below[.](#func.bind.bind-4.sentence-3)

[5](#func.bind.bind-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13466)

*Throws*: Any exception thrown by the initialization of
the state entities of g[.](#func.bind.bind-5.sentence-1)

[6](#func.bind.bind-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13471)

[*Note [1](#func.bind.bind-note-1)*:

If all of FD and TDi meet
the requirements of [*Cpp17CopyConstructible*](utility.arg.requirements#:Cpp17CopyConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]"), then
the return type meets the requirements of [*Cpp17CopyConstructible*](utility.arg.requirements#:Cpp17CopyConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]")[.](#func.bind.bind-6.sentence-1)

— *end note*]

[7](#func.bind.bind-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13479)

The values of the [*bound arguments*](#def:bound_arguments) v1, v2, …, vN and their
corresponding types V1, V2, …, VN depend on the
types TDi derived from
the call to bind and the
cv-qualifiers cv of the call wrapper g as follows:

- [(7.1)](#func.bind.bind-7.1)

if TDi is reference_wrapper<T>, the
argument is tdi.get() and its type Vi is T&;

- [(7.2)](#func.bind.bind-7.2)

if the value of is_bind_expression_v<TDi> is true, the argument isstatic_cast<cv TDi&>(tdi)(std::forward<Uj>(uj)...) and its type Vi isinvoke_result_t<cv TDi&, Uj...>&&;

- [(7.3)](#func.bind.bind-7.3)

if the value j of is_placeholder_v<TDi> is not zero, the argument is std​::​forward<Uj>(uj) and its type Vi is Uj&&;

- [(7.4)](#func.bind.bind-7.4)

otherwise, the value is tdi and its type Vi is cv TDi&[.](#func.bind.bind-7.sentence-1)

[8](#func.bind.bind-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13507)

The value of the target argument vfd is fd and
its corresponding type Vfd is cv FD&[.](#func.bind.bind-8.sentence-1)

#### [22.10.15.5](#func.bind.place) Placeholders [[func.bind.place]](func.bind.place)

namespace std::placeholders {// *M* is the number of placeholders*see below* _1; *see below* _2;
 ⋮ *see below* _*M*;}

[1](#func.bind.place-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13539)

The number *M* of placeholders isimplementation-defined[.](#func.bind.place-1.sentence-1)

[2](#func.bind.place-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13543)

All placeholder types meet the [*Cpp17DefaultConstructible*](utility.arg.requirements#:Cpp17DefaultConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") and[*Cpp17CopyConstructible*](utility.arg.requirements#:Cpp17CopyConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements, and
their default constructors and copy/move
constructors are constexpr functions that
do not throw exceptions[.](#func.bind.place-2.sentence-1)

It is implementation-defined whether
placeholder types meet the [*Cpp17CopyAssignable*](utility.arg.requirements#:Cpp17CopyAssignable "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements,
but if so, their copy assignment operators are
constexpr functions that do not throw exceptions[.](#func.bind.place-2.sentence-2)

[3](#func.bind.place-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13554)

Placeholders should be defined as:inline constexpr *unspecified* _1{};

If they are not, they are declared as:extern *unspecified* _1;

[4](#func.bind.place-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13565)

Placeholders are freestanding items ([[freestanding.item]](freestanding.item "16.3.3.7 Freestanding items"))[.](#func.bind.place-4.sentence-1)

### [22.10.16](#func.memfn) Function template mem_fn [[func.memfn]](func.memfn)

[🔗](#lib:mem_fn)

`template<class R, class T> constexpr unspecified mem_fn(R T::* pm) noexcept;
`

[1](#func.memfn-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13578)

*Returns*: A simple call wrapper ([[func.require]](#func.require#term.simple.call.wrapper "22.10.4 Requirements")) fn with call pattern invoke(pmd, call_args...), wherepmd is the target object of fn of type R T​::​* direct-non-list-initialized with pm, andcall_args is an argument pack
used in a function call expression ([[expr.call]](expr.call "7.6.1.3 Function call")) of fn[.](#func.memfn-1.sentence-1)

### [22.10.17](#func.wrap) Polymorphic function wrappers [[func.wrap]](func.wrap)

#### [22.10.17.1](#func.wrap.general) General [[func.wrap.general]](func.wrap.general)

[1](#func.wrap.general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13594)

Subclause [[func.wrap]](#func.wrap "22.10.17 Polymorphic function wrappers") describes polymorphic wrapper classes that
encapsulate arbitrary callable objects[.](#func.wrap.general-1.sentence-1)

[2](#func.wrap.general-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13598)

Let t be an object of a type that is a specialization offunction, copyable_function, or move_only_function,
such that the target object x of t has a type that
is a specialization offunction, copyable_function, or move_only_function[.](#func.wrap.general-2.sentence-1)

Each argument of the
invocation of x evaluated as part of the invocation of t may alias an argument in the same position in the invocation of t that
has the same type, even if the corresponding parameter is not of reference type[.](#func.wrap.general-2.sentence-2)

[*Example [1](#func.wrap.general-example-1)*: move_only_function<void(T)> f{copyable_function<void(T)>{[](T) {}}};
T t;
f(t); // it is unspecified how many copies of T are made — *end example*]

[3](#func.wrap.general-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13616)

*Recommended practice*: Implementations should avoid double wrapping when
constructing polymorphic wrappers from one another[.](#func.wrap.general-3.sentence-1)

#### [22.10.17.2](#func.wrap.badcall) Class bad_function_call [[func.wrap.badcall]](func.wrap.badcall)

[1](#func.wrap.badcall-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13624)

An exception of type bad_function_call is thrown byfunction​::​operator() ([[func.wrap.func.inv]](#func.wrap.func.inv "22.10.17.3.5 Invocation"))
when the function wrapper object has no target[.](#func.wrap.badcall-1.sentence-1)

namespace std {class bad_function_call : public exception {public:// see [[exception]](exception "17.9.3 Class exception") for the specification of the special member functionsconst char* what() const noexcept override; };}

[🔗](#lib:what,bad_function_call)

`const char* what() const noexcept override;
`

[2](#func.wrap.badcall-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13645)

*Returns*: Animplementation-defined ntbs[.](#func.wrap.badcall-2.sentence-1)

#### [22.10.17.3](#func.wrap.func) Class template function [[func.wrap.func]](func.wrap.func)

#### [22.10.17.3.1](#func.wrap.func.general) General [[func.wrap.func.general]](func.wrap.func.general)

[🔗](#lib:result_type,function)

namespace std {template<class R, class... ArgTypes>class function<R(ArgTypes...)> {public:using result_type = R; // [[func.wrap.func.con]](#func.wrap.func.con "22.10.17.3.2 Constructors and destructor"), construct/copy/destroy function() noexcept;
 function(nullptr_t) noexcept;
 function(const function&);
 function(function&&) noexcept; template<class F> function(F&&);

 function& operator=(const function&);
 function& operator=(function&&);
 function& operator=(nullptr_t) noexcept; template<class F> function& operator=(F&&); template<class F> function& operator=(reference_wrapper<F>) noexcept; ~function(); // [[func.wrap.func.mod]](#func.wrap.func.mod "22.10.17.3.3 Modifiers"), function modifiersvoid swap(function&) noexcept; // [[func.wrap.func.cap]](#func.wrap.func.cap "22.10.17.3.4 Capacity"), function capacityexplicit operator bool() const noexcept; // [[func.wrap.func.inv]](#func.wrap.func.inv "22.10.17.3.5 Invocation"), function invocation R operator()(ArgTypes...) const; // [[func.wrap.func.targ]](#func.wrap.func.targ "22.10.17.3.6 Target access"), function target accessconst type_info& target_type() const noexcept; template<class T> T* target() noexcept; template<class T> const T* target() const noexcept; }; template<class R, class... ArgTypes> function(R(*)(ArgTypes...)) -> function<R(ArgTypes...)>; template<class F> function(F) -> function<*see below*>;}

[1](#func.wrap.func.general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13701)

The function class template provides polymorphic wrappers that
generalize the notion of a function pointer[.](#func.wrap.func.general-1.sentence-1)

Wrappers can store, copy,
and call arbitrary callable objects ([[func.def]](#func.def "22.10.3 Definitions")), given a call
signature ([[func.def]](#func.def "22.10.3 Definitions"))[.](#func.wrap.func.general-1.sentence-2)

[2](#func.wrap.func.general-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13707)

The function class template is a call
wrapper ([[func.def]](#func.def "22.10.3 Definitions")) whose call signature ([[func.def]](#func.def "22.10.3 Definitions"))
is R(ArgTypes...)[.](#func.wrap.func.general-2.sentence-1)

[3](#func.wrap.func.general-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13712)

[*Note [1](#func.wrap.func.general-note-1)*:

The types deduced by the deduction guides for function might change in future revisions of C++[.](#func.wrap.func.general-3.sentence-1)

— *end note*]

#### [22.10.17.3.2](#func.wrap.func.con) Constructors and destructor [[func.wrap.func.con]](func.wrap.func.con)

[🔗](#lib:function,constructor)

`function() noexcept;
`

[1](#func.wrap.func.con-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13726)

*Postconditions*: !*this[.](#func.wrap.func.con-1.sentence-1)

[🔗](#lib:function,constructor_)

`function(nullptr_t) noexcept;
`

[2](#func.wrap.func.con-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13737)

*Postconditions*: !*this[.](#func.wrap.func.con-2.sentence-1)

[🔗](#lib:function,constructor__)

`function(const function& f);
`

[3](#func.wrap.func.con-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13748)

*Postconditions*: !*this if !f; otherwise,
the target object of *this is a copy of the target object of f[.](#func.wrap.func.con-3.sentence-1)

[4](#func.wrap.func.con-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13753)

*Throws*: Nothing if f's target is
a specialization of reference_wrapper or
a function pointer[.](#func.wrap.func.con-4.sentence-1)

Otherwise, may throw bad_alloc or any exception thrown by the copy constructor of the stored callable object[.](#func.wrap.func.con-4.sentence-2)

[5](#func.wrap.func.con-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13760)

*Recommended practice*: Implementations should avoid the use of
dynamically allocated memory for small callable objects, for example, wheref's target is an object holding only a pointer or reference
to an object and a member function pointer[.](#func.wrap.func.con-5.sentence-1)

[🔗](#lib:function,constructor___)

`function(function&& f) noexcept;
`

[6](#func.wrap.func.con-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13774)

*Postconditions*: If !f, *this has no target;
otherwise, the target of *this is equivalent to
the target of f before the construction, andf is in a valid state with an unspecified value[.](#func.wrap.func.con-6.sentence-1)

[7](#func.wrap.func.con-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13781)

*Recommended practice*: Implementations should avoid the use of
dynamically allocated memory for small callable objects, for example,
where f's target is an object holding only a pointer or reference
to an object and a member function pointer[.](#func.wrap.func.con-7.sentence-1)

[🔗](#lib:function,constructor____)

`template<class F> function(F&& f);
`

[8](#func.wrap.func.con-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13795)

Let FD be decay_t<F>[.](#func.wrap.func.con-8.sentence-1)

[9](#func.wrap.func.con-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13798)

*Constraints*: 

- [(9.1)](#func.wrap.func.con-9.1)

is_same_v<remove_cvref_t<F>, function> is false, and

- [(9.2)](#func.wrap.func.con-9.2)

is_invocable_r_v<R, FD&, ArgTypes...> is true[.](#func.wrap.func.con-9.sentence-1)

[10](#func.wrap.func.con-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13807)

*Mandates*: 

- [(10.1)](#func.wrap.func.con-10.1)

is_copy_constructible_v<FD> is true, and

- [(10.2)](#func.wrap.func.con-10.2)

is_constructible_v<FD, F> is true[.](#func.wrap.func.con-10.sentence-1)

[11](#func.wrap.func.con-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13816)

*Preconditions*: FD meets the [*Cpp17CopyConstructible*](utility.arg.requirements#:Cpp17CopyConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements[.](#func.wrap.func.con-11.sentence-1)

[12](#func.wrap.func.con-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13820)

*Postconditions*: !*this is true if any of the following hold:

- [(12.1)](#func.wrap.func.con-12.1)

  f is a null function pointer value[.](#func.wrap.func.con-12.1.sentence-1)

- [(12.2)](#func.wrap.func.con-12.2)

  f is a null member pointer value[.](#func.wrap.func.con-12.2.sentence-1)

- [(12.3)](#func.wrap.func.con-12.3)

  remove_cvref_t<F> is
a specialization of the function class template, and!f is true[.](#func.wrap.func.con-12.3.sentence-1)

[13](#func.wrap.func.con-13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13831)

Otherwise, *this has a target object of type FD direct-non-list-initialized with std​::​forward<F>(f)[.](#func.wrap.func.con-13.sentence-1)

[14](#func.wrap.func.con-14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13835)

*Throws*: Nothing if FD is
a specialization of reference_wrapper or
a function pointer type[.](#func.wrap.func.con-14.sentence-1)

Otherwise, may throw bad_alloc or
any exception thrown by the initialization of the target object[.](#func.wrap.func.con-14.sentence-2)

[15](#func.wrap.func.con-15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13843)

*Recommended practice*: Implementations should avoid the use of
dynamically allocated memory for small callable objects, for example,
where f refers to an object holding only a pointer or
reference to an object and a member function pointer[.](#func.wrap.func.con-15.sentence-1)

[🔗](#func.wrap.func.con-itemdecl:6)

`template<class F> function(F) -> function<see below>;
`

[16](#func.wrap.func.con-16)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13857)

*Constraints*: &F​::​operator() is well-formed when treated as an unevaluated operand and either

- [(16.1)](#func.wrap.func.con-16.1)

F​::​operator() is a non-static member function anddecltype(​&F​::​operator()) is either of the formR(G​::​*)(A...) cv &opt noexceptopt or of the formR(*)(G, A...) noexceptopt for a type G, or

- [(16.2)](#func.wrap.func.con-16.2)

F​::​operator() is a static member function anddecltype(&F​::​operator()) is of the formR(*)(A...) noexceptopt[.](#func.wrap.func.con-16.sentence-1)

[17](#func.wrap.func.con-17)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13874)

*Remarks*: The deduced type is function<R(A...)>[.](#func.wrap.func.con-17.sentence-1)

[18](#func.wrap.func.con-18)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13878)

[*Example [1](#func.wrap.func.con-example-1)*: void f() {int i{5};
 function g = [&](double) { return i; }; // deduces function<int(double)>} — *end example*]

[🔗](#lib:operator=,function)

`function& operator=(const function& f);
`

[19](#func.wrap.func.con-19)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13895)

*Effects*: As if by function(f).swap(*this);

[20](#func.wrap.func.con-20)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13899)

*Returns*: *this[.](#func.wrap.func.con-20.sentence-1)

[🔗](#lib:operator=,function_)

`function& operator=(function&& f);
`

[21](#func.wrap.func.con-21)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13910)

*Effects*: Replaces the target of *this with the target of f[.](#func.wrap.func.con-21.sentence-1)

[22](#func.wrap.func.con-22)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13915)

*Returns*: *this[.](#func.wrap.func.con-22.sentence-1)

[🔗](#lib:operator=,function__)

`function& operator=(nullptr_t) noexcept;
`

[23](#func.wrap.func.con-23)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13926)

*Effects*: If *this != nullptr, destroys the target of this[.](#func.wrap.func.con-23.sentence-1)

[24](#func.wrap.func.con-24)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13930)

*Postconditions*: !(*this)[.](#func.wrap.func.con-24.sentence-1)

[25](#func.wrap.func.con-25)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13934)

*Returns*: *this[.](#func.wrap.func.con-25.sentence-1)

[🔗](#lib:operator=,function___)

`template<class F> function& operator=(F&& f);
`

[26](#func.wrap.func.con-26)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13945)

*Constraints*: is_invocable_r_v<R, decay_t<F>&, ArgTypes...> is true[.](#func.wrap.func.con-26.sentence-1)

[27](#func.wrap.func.con-27)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13949)

*Effects*: As if by: function(std​::​forward<F>(f)).swap(*this);

[28](#func.wrap.func.con-28)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13953)

*Returns*: *this[.](#func.wrap.func.con-28.sentence-1)

[🔗](#lib:operator=,function____)

`template<class F> function& operator=(reference_wrapper<F> f) noexcept;
`

[29](#func.wrap.func.con-29)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13964)

*Effects*: As if by: function(f).swap(*this);

[30](#func.wrap.func.con-30)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13968)

*Returns*: *this[.](#func.wrap.func.con-30.sentence-1)

[🔗](#lib:function,destructor)

`~function();
`

[31](#func.wrap.func.con-31)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13979)

*Effects*: If *this != nullptr, destroys the target of this[.](#func.wrap.func.con-31.sentence-1)

#### [22.10.17.3.3](#func.wrap.func.mod) Modifiers [[func.wrap.func.mod]](func.wrap.func.mod)

[🔗](#lib:swap,function)

`void swap(function& other) noexcept;
`

[1](#func.wrap.func.mod-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13992)

*Effects*: Interchanges the target objects of *this and other[.](#func.wrap.func.mod-1.sentence-1)

#### [22.10.17.3.4](#func.wrap.func.cap) Capacity [[func.wrap.func.cap]](func.wrap.func.cap)

[🔗](#lib:operator_bool,function)

`explicit operator bool() const noexcept;
`

[1](#func.wrap.func.cap-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14005)

*Returns*: true if *this has a target, otherwise false[.](#func.wrap.func.cap-1.sentence-1)

#### [22.10.17.3.5](#func.wrap.func.inv) Invocation [[func.wrap.func.inv]](func.wrap.func.inv)

[🔗](#lib:function,invocation)

`R operator()(ArgTypes... args) const;
`

[1](#func.wrap.func.inv-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14019)

*Returns*: *INVOKE*<R>(f, std​::​forward<ArgTypes>(args)...) ([[func.require]](#func.require "22.10.4 Requirements")),
where f is the target object ([[func.def]](#func.def "22.10.3 Definitions")) of *this[.](#func.wrap.func.inv-1.sentence-1)

[2](#func.wrap.func.inv-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14024)

*Throws*: bad_function_call if !*this; otherwise, any
exception thrown by the target object[.](#func.wrap.func.inv-2.sentence-1)

#### [22.10.17.3.6](#func.wrap.func.targ) Target access [[func.wrap.func.targ]](func.wrap.func.targ)

[🔗](#lib:target_type,function)

`const type_info& target_type() const noexcept;
`

[1](#func.wrap.func.targ-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14038)

*Returns*: If *this has a target of type T, typeid(T); otherwise, typeid(void)[.](#func.wrap.func.targ-1.sentence-1)

[🔗](#lib:target,function)

`template<class T>       T* target() noexcept;
template<class T> const T* target() const noexcept;
`

[2](#func.wrap.func.targ-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14051)

*Returns*: If target_type() == typeid(T) a pointer to the stored function target; otherwise a null pointer[.](#func.wrap.func.targ-2.sentence-1)

#### [22.10.17.3.7](#func.wrap.func.nullptr) Null pointer comparison operator functions [[func.wrap.func.nullptr]](func.wrap.func.nullptr)

[🔗](#lib:operator==,function)

`template<class R, class... ArgTypes>
  bool operator==(const function<R(ArgTypes...)>& f, nullptr_t) noexcept;
`

[1](#func.wrap.func.nullptr-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14066)

*Returns*: !f[.](#func.wrap.func.nullptr-1.sentence-1)

#### [22.10.17.3.8](#func.wrap.func.alg) Specialized algorithms [[func.wrap.func.alg]](func.wrap.func.alg)

[🔗](#lib:swap,function_)

`template<class R, class... ArgTypes>
  void swap(function<R(ArgTypes...)>& f1, function<R(ArgTypes...)>& f2) noexcept;
`

[1](#func.wrap.func.alg-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14080)

*Effects*: As if by: f1.swap(f2);

#### [22.10.17.4](#func.wrap.move) Move-only wrapper [[func.wrap.move]](func.wrap.move)

#### [22.10.17.4.1](#func.wrap.move.general) General [[func.wrap.move.general]](func.wrap.move.general)

[1](#func.wrap.move.general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14089)

The header provides partial specializations of move_only_function for each combination of the possible replacements
of the placeholders cv, *ref*, and *noex* where

- [(1.1)](#func.wrap.move.general-1.1)

cv is either const or empty,

- [(1.2)](#func.wrap.move.general-1.2)

*ref* is either &, &&, or empty, and

- [(1.3)](#func.wrap.move.general-1.3)

*noex* is either true or false[.](#func.wrap.move.general-1.sentence-1)

[2](#func.wrap.move.general-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14102)

For each of the possible combinations of the placeholders mentioned above,
there is a placeholder *inv-quals* defined as follows:

- [(2.1)](#func.wrap.move.general-2.1)

If *ref* is empty, let *inv-quals* be cv&,

- [(2.2)](#func.wrap.move.general-2.2)

otherwise, let *inv-quals* be cv *ref*[.](#func.wrap.move.general-2.sentence-1)

#### [22.10.17.4.2](#func.wrap.move.class) Class template move_only_function [[func.wrap.move.class]](func.wrap.move.class)

[🔗](#lib:move_only_function)

namespace std {template<class R, class... ArgTypes>class move_only_function<R(ArgTypes...) cv *ref* noexcept(*noex*)> {public:using result_type = R; // [[func.wrap.move.ctor]](#func.wrap.move.ctor "22.10.17.4.3 Constructors, assignments, and destructor"), constructors, assignments, and destructor move_only_function() noexcept;
 move_only_function(nullptr_t) noexcept;
 move_only_function(move_only_function&&) noexcept; template<class F> move_only_function(F&&); template<class T, class... Args>explicit move_only_function(in_place_type_t<T>, Args&&...); template<class T, class U, class... Args>explicit move_only_function(in_place_type_t<T>, initializer_list<U>, Args&&...);

 move_only_function& operator=(move_only_function&&);
 move_only_function& operator=(nullptr_t) noexcept; template<class F> move_only_function& operator=(F&&); ~move_only_function(); // [[func.wrap.move.inv]](#func.wrap.move.inv "22.10.17.4.4 Invocation"), invocationexplicit operator bool() const noexcept;
 R operator()(ArgTypes...) cv *ref* noexcept(*noex*); // [[func.wrap.move.util]](#func.wrap.move.util "22.10.17.4.5 Utility"), utilityvoid swap(move_only_function&) noexcept; friend void swap(move_only_function&, move_only_function&) noexcept; friend bool operator==(const move_only_function&, nullptr_t) noexcept; private:template<class VT>static constexpr bool *is-callable-from* = *see below*; // *exposition only*};}

[1](#func.wrap.move.class-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14154)

The move_only_function class template provides polymorphic wrappers
that generalize the notion of a callable object ([[func.def]](#func.def "22.10.3 Definitions"))[.](#func.wrap.move.class-1.sentence-1)

These wrappers can store, move, and call arbitrary callable objects,
given a call signature[.](#func.wrap.move.class-1.sentence-2)

[2](#func.wrap.move.class-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14160)

*Recommended practice*: Implementations should avoid the use of dynamically allocated memory
for a small contained value[.](#func.wrap.move.class-2.sentence-1)

[*Note [1](#func.wrap.move.class-note-1)*:

Such small-object optimization can only be applied to a type T for which is_nothrow_move_constructible_v<T> is true[.](#func.wrap.move.class-2.sentence-2)

— *end note*]

#### [22.10.17.4.3](#func.wrap.move.ctor) Constructors, assignments, and destructor [[func.wrap.move.ctor]](func.wrap.move.ctor)

[🔗](#:move_only_function::is-callable-from)

`template<class VT>
  static constexpr bool is-callable-from = see below;
`

[1](#func.wrap.move.ctor-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14178)

If *noex* is true,*is-callable-from*<VT> is equal to:is_nothrow_invocable_r_v<R, VT cv *ref*, ArgTypes...> && is_nothrow_invocable_r_v<R, VT *inv-quals*, ArgTypes...>

Otherwise, *is-callable-from*<VT> is equal to:is_invocable_r_v<R, VT cv *ref*, ArgTypes...> && is_invocable_r_v<R, VT *inv-quals*, ArgTypes...>

[🔗](#lib:move_only_function,constructor)

`move_only_function() noexcept;
move_only_function(nullptr_t) noexcept;
`

[2](#func.wrap.move.ctor-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14199)

*Postconditions*: *this has no target object[.](#func.wrap.move.ctor-2.sentence-1)

[🔗](#lib:move_only_function,constructor_)

`move_only_function(move_only_function&& f) noexcept;
`

[3](#func.wrap.move.ctor-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14210)

*Postconditions*: The target object of *this is
the target object f had before construction, andf is in a valid state with an unspecified value[.](#func.wrap.move.ctor-3.sentence-1)

[🔗](#lib:move_only_function,constructor__)

`template<class F> move_only_function(F&& f);
`

[4](#func.wrap.move.ctor-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14223)

Let VT be decay_t<F>[.](#func.wrap.move.ctor-4.sentence-1)

[5](#func.wrap.move.ctor-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14226)

*Constraints*: 

- [(5.1)](#func.wrap.move.ctor-5.1)

remove_cvref_t<F> is not the same type as move_only_function, and

- [(5.2)](#func.wrap.move.ctor-5.2)

remove_cvref_t<F> is not a specialization of in_place_type_t, and

- [(5.3)](#func.wrap.move.ctor-5.3)

*is-callable-from*<VT> is true[.](#func.wrap.move.ctor-5.sentence-1)

[6](#func.wrap.move.ctor-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14237)

*Mandates*: is_constructible_v<VT, F> is true[.](#func.wrap.move.ctor-6.sentence-1)

[7](#func.wrap.move.ctor-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14241)

*Preconditions*: VT meets the [*Cpp17Destructible*](utility.arg.requirements#:Cpp17Destructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements, and
if is_move_constructible_v<VT> is true,VT meets the [*Cpp17MoveConstructible*](utility.arg.requirements#:Cpp17MoveConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements[.](#func.wrap.move.ctor-7.sentence-1)

[8](#func.wrap.move.ctor-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14247)

*Postconditions*: *this has no target object if any of the following hold:

- [(8.1)](#func.wrap.move.ctor-8.1)

f is a null function pointer value, or

- [(8.2)](#func.wrap.move.ctor-8.2)

f is a null member pointer value, or

- [(8.3)](#func.wrap.move.ctor-8.3)

remove_cvref_t<F> is a specialization of
the move_only_function class template,
and f has no target object[.](#func.wrap.move.ctor-8.sentence-1)

Otherwise, *this has a target object of type VT direct-non-list-initialized with std​::​forward<F>(f)[.](#func.wrap.move.ctor-8.sentence-2)

[9](#func.wrap.move.ctor-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14263)

*Throws*: Any exception thrown by the initialization of the target object[.](#func.wrap.move.ctor-9.sentence-1)

May throw bad_alloc unless VT is
a function pointer or a specialization of reference_wrapper[.](#func.wrap.move.ctor-9.sentence-2)

[🔗](#lib:move_only_function,constructor___)

`template<class T, class... Args>
  explicit move_only_function(in_place_type_t<T>, Args&&... args);
`

[10](#func.wrap.move.ctor-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14277)

Let VT be decay_t<T>[.](#func.wrap.move.ctor-10.sentence-1)

[11](#func.wrap.move.ctor-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14280)

*Constraints*: 

- [(11.1)](#func.wrap.move.ctor-11.1)

is_constructible_v<VT, Args...> is true, and

- [(11.2)](#func.wrap.move.ctor-11.2)

*is-callable-from*<VT> is true[.](#func.wrap.move.ctor-11.sentence-1)

[12](#func.wrap.move.ctor-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14289)

*Mandates*: VT is the same type as T[.](#func.wrap.move.ctor-12.sentence-1)

[13](#func.wrap.move.ctor-13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14293)

*Preconditions*: VT meets the [*Cpp17Destructible*](utility.arg.requirements#:Cpp17Destructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements, and
if is_move_constructible_v<VT> is true,VT meets the [*Cpp17MoveConstructible*](utility.arg.requirements#:Cpp17MoveConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements[.](#func.wrap.move.ctor-13.sentence-1)

[14](#func.wrap.move.ctor-14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14299)

*Postconditions*: *this has a target object of type VT direct-non-list-initialized with std​::​forward<Args>​(args)...[.](#func.wrap.move.ctor-14.sentence-1)

[15](#func.wrap.move.ctor-15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14304)

*Throws*: Any exception thrown by the initialization of the target object[.](#func.wrap.move.ctor-15.sentence-1)

May throw bad_alloc unless VT is
a function pointer or a specialization of reference_wrapper[.](#func.wrap.move.ctor-15.sentence-2)

[🔗](#lib:move_only_function,constructor____)

`template<class T, class U, class... Args>
  explicit move_only_function(in_place_type_t<T>, initializer_list<U> ilist, Args&&... args);
`

[16](#func.wrap.move.ctor-16)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14318)

Let VT be decay_t<T>[.](#func.wrap.move.ctor-16.sentence-1)

[17](#func.wrap.move.ctor-17)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14321)

*Constraints*: 

- [(17.1)](#func.wrap.move.ctor-17.1)

is_constructible_v<VT, initializer_list<U>&, Args...> istrue, and

- [(17.2)](#func.wrap.move.ctor-17.2)

*is-callable-from*<VT> is true[.](#func.wrap.move.ctor-17.sentence-1)

[18](#func.wrap.move.ctor-18)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14331)

*Mandates*: VT is the same type as T[.](#func.wrap.move.ctor-18.sentence-1)

[19](#func.wrap.move.ctor-19)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14335)

*Preconditions*: VT meets the [*Cpp17Destructible*](utility.arg.requirements#:Cpp17Destructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements, and
if is_move_constructible_v<VT> is true,VT meets the [*Cpp17MoveConstructible*](utility.arg.requirements#:Cpp17MoveConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements[.](#func.wrap.move.ctor-19.sentence-1)

[20](#func.wrap.move.ctor-20)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14341)

*Postconditions*: *this has a target object of type VT direct-non-list-initialized withilist, std​::​forward<Args>(args)...[.](#func.wrap.move.ctor-20.sentence-1)

[21](#func.wrap.move.ctor-21)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14347)

*Throws*: Any exception thrown by the initialization of the target object[.](#func.wrap.move.ctor-21.sentence-1)

May throw bad_alloc unless VT is
a function pointer or a specialization of reference_wrapper[.](#func.wrap.move.ctor-21.sentence-2)

[🔗](#lib:operator=,move_only_function)

`move_only_function& operator=(move_only_function&& f);
`

[22](#func.wrap.move.ctor-22)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14360)

*Effects*: Equivalent to: move_only_function(std​::​move(f)).swap(*this);

[23](#func.wrap.move.ctor-23)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14364)

*Returns*: *this[.](#func.wrap.move.ctor-23.sentence-1)

[🔗](#lib:operator=,move_only_function_)

`move_only_function& operator=(nullptr_t) noexcept;
`

[24](#func.wrap.move.ctor-24)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14375)

*Effects*: Destroys the target object of *this, if any[.](#func.wrap.move.ctor-24.sentence-1)

[25](#func.wrap.move.ctor-25)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14379)

*Returns*: *this[.](#func.wrap.move.ctor-25.sentence-1)

[🔗](#lib:operator=,move_only_function__)

`template<class F> move_only_function& operator=(F&& f);
`

[26](#func.wrap.move.ctor-26)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14390)

*Effects*: Equivalent to: move_only_function(std​::​forward<F>(f)).swap(*this);

[27](#func.wrap.move.ctor-27)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14394)

*Returns*: *this[.](#func.wrap.move.ctor-27.sentence-1)

[🔗](#lib:move_only_function,destructor)

`~move_only_function();
`

[28](#func.wrap.move.ctor-28)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14405)

*Effects*: Destroys the target object of *this, if any[.](#func.wrap.move.ctor-28.sentence-1)

#### [22.10.17.4.4](#func.wrap.move.inv) Invocation [[func.wrap.move.inv]](func.wrap.move.inv)

[🔗](#lib:operator_bool,move_only_function)

`explicit operator bool() const noexcept;
`

[1](#func.wrap.move.inv-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14418)

*Returns*: true if *this has a target object, otherwise false[.](#func.wrap.move.inv-1.sentence-1)

[🔗](#lib:operator(),move_only_function)

`R operator()(ArgTypes... args) cv ref noexcept(noex);
`

[2](#func.wrap.move.inv-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14429)

*Preconditions*: *this has a target object[.](#func.wrap.move.inv-2.sentence-1)

[3](#func.wrap.move.inv-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14433)

*Effects*: Equivalent to:return *INVOKE*<R>(static_cast<F *inv-quals*>(f), std::forward<ArgTypes>(args)...); where f is an lvalue designating the target object of *this andF is the type of f[.](#func.wrap.move.inv-3.sentence-1)

#### [22.10.17.4.5](#func.wrap.move.util) Utility [[func.wrap.move.util]](func.wrap.move.util)

[🔗](#lib:swap,move_only_function)

`void swap(move_only_function& other) noexcept;
`

[1](#func.wrap.move.util-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14451)

*Effects*: Exchanges the target objects of *this and other[.](#func.wrap.move.util-1.sentence-1)

[🔗](#lib:swap,move_only_function_)

`friend void swap(move_only_function& f1, move_only_function& f2) noexcept;
`

[2](#func.wrap.move.util-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14462)

*Effects*: Equivalent to f1.swap(f2)[.](#func.wrap.move.util-2.sentence-1)

[🔗](#lib:operator==,move_only_function)

`friend bool operator==(const move_only_function& f, nullptr_t) noexcept;
`

[3](#func.wrap.move.util-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14473)

*Returns*: true if f has no target object, otherwise false[.](#func.wrap.move.util-3.sentence-1)

#### [22.10.17.5](#func.wrap.copy) Copyable wrapper [[func.wrap.copy]](func.wrap.copy)

#### [22.10.17.5.1](#func.wrap.copy.general) General [[func.wrap.copy.general]](func.wrap.copy.general)

[1](#func.wrap.copy.general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14482)

The header provides partial specializations of copyable_function for each combination of the possible replacements
of the placeholders cv, *ref*, and *noex* where

- [(1.1)](#func.wrap.copy.general-1.1)

cv is either const or empty,

- [(1.2)](#func.wrap.copy.general-1.2)

*ref* is either &, &&, or empty, and

- [(1.3)](#func.wrap.copy.general-1.3)

*noex* is either true or false[.](#func.wrap.copy.general-1.sentence-1)

[2](#func.wrap.copy.general-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14495)

For each of the possible combinations of the placeholders mentioned above,
there is a placeholder *inv-quals* defined as follows:

- [(2.1)](#func.wrap.copy.general-2.1)

If *ref* is empty, let *inv-quals* be cv&,

- [(2.2)](#func.wrap.copy.general-2.2)

otherwise, let *inv-quals* be cv *ref*[.](#func.wrap.copy.general-2.sentence-1)

#### [22.10.17.5.2](#func.wrap.copy.class) Class template copyable_function [[func.wrap.copy.class]](func.wrap.copy.class)

[🔗](#lib:copyable_function)

namespace std {template<class R, class... ArgTypes>class copyable_function<R(ArgTypes...) cv *ref* noexcept(*noex*)> {public:using result_type = R; // [[func.wrap.copy.ctor]](#func.wrap.copy.ctor "22.10.17.5.3 Constructors, assignments, and destructor"), constructors, assignments, and destructor copyable_function() noexcept;
 copyable_function(nullptr_t) noexcept;
 copyable_function(const copyable_function&);
 copyable_function(copyable_function&&) noexcept; template<class F> copyable_function(F&&); template<class T, class... Args>explicit copyable_function(in_place_type_t<T>, Args&&...); template<class T, class U, class... Args>explicit copyable_function(in_place_type_t<T>, initializer_list<U>, Args&&...);

 copyable_function& operator=(const copyable_function&);
 copyable_function& operator=(copyable_function&&);
 copyable_function& operator=(nullptr_t) noexcept; template<class F> copyable_function& operator=(F&&); ~copyable_function(); // [[func.wrap.copy.inv]](#func.wrap.copy.inv "22.10.17.5.4 Invocation"), invocationexplicit operator bool() const noexcept;
 R operator()(ArgTypes...) cv *ref* noexcept(*noex*); // [[func.wrap.copy.util]](#func.wrap.copy.util "22.10.17.5.5 Utility"), utilityvoid swap(copyable_function&) noexcept; friend void swap(copyable_function&, copyable_function&) noexcept; friend bool operator==(const copyable_function&, nullptr_t) noexcept; private:template<class VT>static constexpr bool *is-callable-from* = *see below*; // *exposition only*};}

[1](#func.wrap.copy.class-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14549)

The copyable_function class template provides polymorphic wrappers
that generalize the notion of a callable object ([[func.def]](#func.def "22.10.3 Definitions"))[.](#func.wrap.copy.class-1.sentence-1)

These wrappers can store, copy, move, and call arbitrary callable objects,
given a call signature[.](#func.wrap.copy.class-1.sentence-2)

[2](#func.wrap.copy.class-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14555)

*Recommended practice*: Implementations should avoid the use of dynamically allocated memory
for a small contained value[.](#func.wrap.copy.class-2.sentence-1)

[*Note [1](#func.wrap.copy.class-note-1)*:

Such small-object optimization can only be applied to a type T for which is_nothrow_move_constructible_v<T> is true[.](#func.wrap.copy.class-2.sentence-2)

— *end note*]

#### [22.10.17.5.3](#func.wrap.copy.ctor) Constructors, assignments, and destructor [[func.wrap.copy.ctor]](func.wrap.copy.ctor)

[🔗](#:copyable_function::is-callable-from)

`template<class VT>
  static constexpr bool is-callable-from = see below;
`

[1](#func.wrap.copy.ctor-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14573)

If *noex* is true,*is-callable-from*<VT> is equal to:is_nothrow_invocable_r_v<R, VT cv *ref*, ArgTypes...> && is_nothrow_invocable_r_v<R, VT *inv-quals*, ArgTypes...>

Otherwise, *is-callable-from*<VT> is equal to:is_invocable_r_v<R, VT cv *ref*, ArgTypes...> && is_invocable_r_v<R, VT *inv-quals*, ArgTypes...>

[🔗](#lib:copyable_function,constructor)

`copyable_function() noexcept;
copyable_function(nullptr_t) noexcept;
`

[2](#func.wrap.copy.ctor-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14594)

*Postconditions*: *this has no target object[.](#func.wrap.copy.ctor-2.sentence-1)

[🔗](#lib:copyable_function,constructor_)

`copyable_function(const copyable_function& f);
`

[3](#func.wrap.copy.ctor-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14605)

*Postconditions*: *this has no target object if f had no target object[.](#func.wrap.copy.ctor-3.sentence-1)

Otherwise, the target object of *this is a copy of the target object of f[.](#func.wrap.copy.ctor-3.sentence-2)

[4](#func.wrap.copy.ctor-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14611)

*Throws*: Any exception thrown by the initialization of the target object[.](#func.wrap.copy.ctor-4.sentence-1)

May throw bad_alloc[.](#func.wrap.copy.ctor-4.sentence-2)

[🔗](#lib:copyable_function,constructor__)

`copyable_function(copyable_function&& f) noexcept;
`

[5](#func.wrap.copy.ctor-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14623)

*Postconditions*: The target object of *this is
the target object f had before construction, andf is in a valid state with an unspecified value[.](#func.wrap.copy.ctor-5.sentence-1)

[🔗](#lib:copyable_function,constructor___)

`template<class F> copyable_function(F&& f);
`

[6](#func.wrap.copy.ctor-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14636)

Let VT be decay_t<F>[.](#func.wrap.copy.ctor-6.sentence-1)

[7](#func.wrap.copy.ctor-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14639)

*Constraints*: 

- [(7.1)](#func.wrap.copy.ctor-7.1)

remove_cvref_t<F> is not the same type as copyable_function, and

- [(7.2)](#func.wrap.copy.ctor-7.2)

remove_cvref_t<F> is not a specialization of in_place_type_t, and

- [(7.3)](#func.wrap.copy.ctor-7.3)

*is-callable-from*<VT> is true[.](#func.wrap.copy.ctor-7.sentence-1)

[8](#func.wrap.copy.ctor-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14650)

*Mandates*: 

- [(8.1)](#func.wrap.copy.ctor-8.1)

is_constructible_v<VT, F> is true, and

- [(8.2)](#func.wrap.copy.ctor-8.2)

is_copy_constructible_v<VT> is true[.](#func.wrap.copy.ctor-8.sentence-1)

[9](#func.wrap.copy.ctor-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14659)

*Preconditions*: VT meets the [*Cpp17Destructible*](utility.arg.requirements#:Cpp17Destructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") and[*Cpp17CopyConstructible*](utility.arg.requirements#:Cpp17CopyConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements[.](#func.wrap.copy.ctor-9.sentence-1)

[10](#func.wrap.copy.ctor-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14664)

*Postconditions*: *this has no target object if any of the following hold:

- [(10.1)](#func.wrap.copy.ctor-10.1)

f is a null function pointer value, or

- [(10.2)](#func.wrap.copy.ctor-10.2)

f is a null member pointer value, or

- [(10.3)](#func.wrap.copy.ctor-10.3)

remove_cvref_t<F> is a specialization of
the copyable_function class template,
and f has no target object[.](#func.wrap.copy.ctor-10.sentence-1)

Otherwise, *this has a target object of type VT direct-non-list-initialized with std​::​forward<F>(f)[.](#func.wrap.copy.ctor-10.sentence-2)

[11](#func.wrap.copy.ctor-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14680)

*Throws*: Any exception thrown by the initialization of the target object[.](#func.wrap.copy.ctor-11.sentence-1)

May throw bad_alloc unless VT is
a function pointer or a specialization of reference_wrapper[.](#func.wrap.copy.ctor-11.sentence-2)

[🔗](#lib:copyable_function,constructor____)

`template<class T, class... Args>
  explicit copyable_function(in_place_type_t<T>, Args&&... args);
`

[12](#func.wrap.copy.ctor-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14694)

Let VT be decay_t<T>[.](#func.wrap.copy.ctor-12.sentence-1)

[13](#func.wrap.copy.ctor-13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14697)

*Constraints*: 

- [(13.1)](#func.wrap.copy.ctor-13.1)

is_constructible_v<VT, Args...> is true, and

- [(13.2)](#func.wrap.copy.ctor-13.2)

*is-callable-from*<VT> is true[.](#func.wrap.copy.ctor-13.sentence-1)

[14](#func.wrap.copy.ctor-14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14706)

*Mandates*: 

- [(14.1)](#func.wrap.copy.ctor-14.1)

VT is the same type as T, and

- [(14.2)](#func.wrap.copy.ctor-14.2)

is_copy_constructible_v<VT> is true[.](#func.wrap.copy.ctor-14.sentence-1)

[15](#func.wrap.copy.ctor-15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14715)

*Preconditions*: VT meets the [*Cpp17Destructible*](utility.arg.requirements#:Cpp17Destructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") and[*Cpp17CopyConstructible*](utility.arg.requirements#:Cpp17CopyConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements[.](#func.wrap.copy.ctor-15.sentence-1)

[16](#func.wrap.copy.ctor-16)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14720)

*Postconditions*: *this has a target object of type VT direct-non-list-initialized with std​::​forward<Args>​(args)...[.](#func.wrap.copy.ctor-16.sentence-1)

[17](#func.wrap.copy.ctor-17)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14725)

*Throws*: Any exception thrown by the initialization of the target object[.](#func.wrap.copy.ctor-17.sentence-1)

May throw bad_alloc unless VT is
a pointer or a specialization of reference_wrapper[.](#func.wrap.copy.ctor-17.sentence-2)

[🔗](#lib:copyable_function,constructor_____)

`template<class T, class U, class... Args>
  explicit copyable_function(in_place_type_t<T>, initializer_list<U> ilist, Args&&... args);
`

[18](#func.wrap.copy.ctor-18)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14739)

Let VT be decay_t<T>[.](#func.wrap.copy.ctor-18.sentence-1)

[19](#func.wrap.copy.ctor-19)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14742)

*Constraints*: 

- [(19.1)](#func.wrap.copy.ctor-19.1)

is_constructible_v<VT, initializer_list<U>&, Args...> istrue, and

- [(19.2)](#func.wrap.copy.ctor-19.2)

*is-callable-from*<VT> is true[.](#func.wrap.copy.ctor-19.sentence-1)

[20](#func.wrap.copy.ctor-20)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14752)

*Mandates*: 

- [(20.1)](#func.wrap.copy.ctor-20.1)

VT is the same type as T, and

- [(20.2)](#func.wrap.copy.ctor-20.2)

is_copy_constructible_v<VT> is true[.](#func.wrap.copy.ctor-20.sentence-1)

[21](#func.wrap.copy.ctor-21)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14761)

*Preconditions*: VT meets the [*Cpp17Destructible*](utility.arg.requirements#:Cpp17Destructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") and[*Cpp17CopyConstructible*](utility.arg.requirements#:Cpp17CopyConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements[.](#func.wrap.copy.ctor-21.sentence-1)

[22](#func.wrap.copy.ctor-22)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14766)

*Postconditions*: *this has a target object of type VT direct-non-list-initialized withilist, std​::​forward<Args>(args)...[.](#func.wrap.copy.ctor-22.sentence-1)

[23](#func.wrap.copy.ctor-23)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14772)

*Throws*: Any exception thrown by the initialization of the target object[.](#func.wrap.copy.ctor-23.sentence-1)

May throw bad_alloc unless VT is
a pointer or a specialization of reference_wrapper[.](#func.wrap.copy.ctor-23.sentence-2)

[🔗](#lib:operator=,copyable_function)

`copyable_function& operator=(const copyable_function& f);
`

[24](#func.wrap.copy.ctor-24)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14785)

*Effects*: Equivalent to: copyable_function(f).swap(*this);

[25](#func.wrap.copy.ctor-25)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14789)

*Returns*: *this[.](#func.wrap.copy.ctor-25.sentence-1)

[🔗](#lib:operator=,copyable_function_)

`copyable_function& operator=(copyable_function&& f);
`

[26](#func.wrap.copy.ctor-26)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14800)

*Effects*: Equivalent to: copyable_function(std​::​move(f)).swap(*this);

[27](#func.wrap.copy.ctor-27)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14804)

*Returns*: *this[.](#func.wrap.copy.ctor-27.sentence-1)

[🔗](#lib:operator=,copyable_function__)

`copyable_function& operator=(nullptr_t) noexcept;
`

[28](#func.wrap.copy.ctor-28)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14815)

*Effects*: Destroys the target object of *this, if any[.](#func.wrap.copy.ctor-28.sentence-1)

[29](#func.wrap.copy.ctor-29)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14819)

*Returns*: *this[.](#func.wrap.copy.ctor-29.sentence-1)

[🔗](#lib:operator=,copyable_function___)

`template<class F> copyable_function& operator=(F&& f);
`

[30](#func.wrap.copy.ctor-30)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14830)

*Effects*: Equivalent to: copyable_function(std​::​forward<F>(f)).swap(*this);

[31](#func.wrap.copy.ctor-31)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14834)

*Returns*: *this[.](#func.wrap.copy.ctor-31.sentence-1)

[🔗](#lib:copyable_function,destructor)

`~copyable_function();
`

[32](#func.wrap.copy.ctor-32)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14845)

*Effects*: Destroys the target object of *this, if any[.](#func.wrap.copy.ctor-32.sentence-1)

#### [22.10.17.5.4](#func.wrap.copy.inv) Invocation [[func.wrap.copy.inv]](func.wrap.copy.inv)

[🔗](#lib:operator_bool,copyable_function)

`explicit operator bool() const noexcept;
`

[1](#func.wrap.copy.inv-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14858)

*Returns*: true if *this has a target object, otherwise false[.](#func.wrap.copy.inv-1.sentence-1)

[🔗](#lib:operator(),copyable_function)

`R operator()(ArgTypes... args) cv ref noexcept(noex);
`

[2](#func.wrap.copy.inv-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14869)

*Preconditions*: *this has a target object[.](#func.wrap.copy.inv-2.sentence-1)

[3](#func.wrap.copy.inv-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14873)

*Effects*: Equivalent to:return *INVOKE*<R>(static_cast<F *inv-quals*>(f), std::forward<ArgTypes>(args)...); where f is an lvalue designating the target object of *this andF is the type of f[.](#func.wrap.copy.inv-3.sentence-1)

#### [22.10.17.5.5](#func.wrap.copy.util) Utility [[func.wrap.copy.util]](func.wrap.copy.util)

[🔗](#lib:swap,copyable_function)

`void swap(copyable_function& other) noexcept;
`

[1](#func.wrap.copy.util-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14891)

*Effects*: Exchanges the target objects of *this and other[.](#func.wrap.copy.util-1.sentence-1)

[🔗](#lib:swap,copyable_function_)

`friend void swap(copyable_function& f1, copyable_function& f2) noexcept;
`

[2](#func.wrap.copy.util-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14902)

*Effects*: Equivalent to f1.swap(f2)[.](#func.wrap.copy.util-2.sentence-1)

[🔗](#lib:operator==,copyable_function)

`friend bool operator==(const copyable_function& f, nullptr_t) noexcept;
`

[3](#func.wrap.copy.util-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14913)

*Returns*: true if f has no target object, otherwise false[.](#func.wrap.copy.util-3.sentence-1)

#### [22.10.17.6](#func.wrap.ref) Non-owning wrapper [[func.wrap.ref]](func.wrap.ref)

#### [22.10.17.6.1](#func.wrap.ref.general) General [[func.wrap.ref.general]](func.wrap.ref.general)

[1](#func.wrap.ref.general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14922)

The header provides partial specializations of function_ref for each combination of the possible replacements of
the placeholders cv and *noex* where:

- [(1.1)](#func.wrap.ref.general-1.1)

cv is either const or empty, and

- [(1.2)](#func.wrap.ref.general-1.2)

*noex* is either true or false[.](#func.wrap.ref.general-1.sentence-1)

#### [22.10.17.6.2](#func.wrap.ref.class) Class template function_ref [[func.wrap.ref.class]](func.wrap.ref.class)

[🔗](#lib:function_ref)

namespace std {template<class R, class... ArgTypes>class function_ref<R(ArgTypes...) cv noexcept(*noex*)> {public:// [[func.wrap.ref.ctor]](#func.wrap.ref.ctor "22.10.17.6.3 Constructors and assignment operators"), constructors and assignment operatorstemplate<class F> function_ref(F*) noexcept; template<class F> constexpr function_ref(F&&) noexcept; template<auto f> constexpr function_ref(nontype_t<f>) noexcept; template<auto f, class U> constexpr function_ref(nontype_t<f>, U&&) noexcept; template<auto f, class T> constexpr function_ref(nontype_t<f>, cv T*) noexcept; constexpr function_ref(const function_ref&) noexcept = default; constexpr function_ref& operator=(const function_ref&) noexcept = default; template<class T> function_ref& operator=(T) = delete; // [[func.wrap.ref.inv]](#func.wrap.ref.inv "22.10.17.6.4 Invocation"), invocation R operator()(ArgTypes...) const noexcept(*noex*); private:template<class... T>static constexpr bool *is-invocable-using* = *see below*; // *exposition only* R (**thunk-ptr*)(*BoundEntityType*, Args&&...) noexcept(*noex*); // *exposition only**BoundEntityType* *bound-entity*; // *exposition only*}; // [[func.wrap.ref.deduct]](#func.wrap.ref.deduct "22.10.17.6.5 Deduction guides"), deduction guidestemplate<class F> function_ref(F*) -> function_ref<F>; template<auto f> function_ref(nontype_t<f>) -> function_ref<*see below*>; template<auto f, class T> function_ref(nontype_t<f>, T&&) -> function_ref<*see below*>;}

[1](#func.wrap.ref.class-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14971)

An object of classfunction_ref<R(Args...) cv noexcept(*noex*)> stores a pointer to function *thunk-ptr* and
an object *bound-entity*[.](#func.wrap.ref.class-1.sentence-1)

*bound-entity* has
an unspecified trivially copyable type *BoundEntityType*, that
models [copyable](concepts.object#concept:copyable "18.6 Object concepts [concepts.object]") and
is capable of storing a pointer to object value or a pointer to function value[.](#func.wrap.ref.class-1.sentence-2)

The type of *thunk-ptr* isR(*)(*BoundEntityType*, Args&&...) noexcept(*noex*)[.](#func.wrap.ref.class-1.sentence-3)

[2](#func.wrap.ref.class-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14983)

Each specialization of function_ref is
a trivially copyable type ([[basic.types.general]](basic.types.general#term.trivially.copyable.type "6.9.1 General"))
that models [copyable](concepts.object#concept:copyable "18.6 Object concepts [concepts.object]")[.](#func.wrap.ref.class-2.sentence-1)

[3](#func.wrap.ref.class-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L14988)

Within [[func.wrap.ref]](#func.wrap.ref "22.10.17.6 Non-owning wrapper"),*call-args* is an argument pack with elements such thatdecltype((*call-args*))... denoteArgs&&... respectively[.](#func.wrap.ref.class-3.sentence-1)

#### [22.10.17.6.3](#func.wrap.ref.ctor) Constructors and assignment operators [[func.wrap.ref.ctor]](func.wrap.ref.ctor)

[🔗](#:function_ref::is-invocable-using)

`template<class... T>
  static constexpr bool is-invocable-using = see below;
`

[1](#func.wrap.ref.ctor-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15003)

If *noex* is true,*is-invocable-using*<T...> is equal to:is_nothrow_invocable_r_v<R, T..., ArgTypes...>

Otherwise, *is-invocable-using*<T...> is equal to:is_invocable_r_v<R, T..., ArgTypes...>

[🔗](#lib:function_ref,constructor)

`template<class F> function_ref(F* f) noexcept;
`

[2](#func.wrap.ref.ctor-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15021)

*Constraints*: 

- [(2.1)](#func.wrap.ref.ctor-2.1)

is_function_v<F> is true, and

- [(2.2)](#func.wrap.ref.ctor-2.2)

*is-invocable-using*<F> is true[.](#func.wrap.ref.ctor-2.sentence-1)

[3](#func.wrap.ref.ctor-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15028)

*Preconditions*: f is not a null pointer[.](#func.wrap.ref.ctor-3.sentence-1)

[4](#func.wrap.ref.ctor-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15032)

*Effects*: Initializes*bound-entity* with f, and*thunk-ptr* with the address of a function *thunk* such that*thunk*(*bound-entity*, *call-args*...) is expression-equivalent ([[defns.expression.equivalent]](defns.expression.equivalent "3.22 expression-equivalent")) toinvoke_r<R>(f, *call-args*...)[.](#func.wrap.ref.ctor-4.sentence-1)

[🔗](#lib:function_ref,constructor_)

`template<class F> constexpr function_ref(F&& f) noexcept;
`

[5](#func.wrap.ref.ctor-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15049)

Let T be remove_reference_t<F>[.](#func.wrap.ref.ctor-5.sentence-1)

[6](#func.wrap.ref.ctor-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15052)

*Constraints*: 

- [(6.1)](#func.wrap.ref.ctor-6.1)

remove_cvref_t<F> is not the same type as function_ref,

- [(6.2)](#func.wrap.ref.ctor-6.2)

is_member_pointer_v<T> is false, and

- [(6.3)](#func.wrap.ref.ctor-6.3)

*is-invocable-using*<cv T&> is true[.](#func.wrap.ref.ctor-6.sentence-1)

[7](#func.wrap.ref.ctor-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15060)

*Effects*: Initializes*bound-entity* with addressof(f), and*thunk-ptr* with the address of a function *thunk* such that*thunk*(*bound-entity*, *call-args*...) is expression-equivalent ([[defns.expression.equivalent]](defns.expression.equivalent "3.22 expression-equivalent")) toinvoke_r<R>(static_cast<cv T&>(f), *call-args*...)[.](#func.wrap.ref.ctor-7.sentence-1)

[🔗](#lib:function_ref,constructor__)

`template<auto f> constexpr function_ref(nontype_t<f>) noexcept;
`

[8](#func.wrap.ref.ctor-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15077)

Let F be decltype(f)[.](#func.wrap.ref.ctor-8.sentence-1)

[9](#func.wrap.ref.ctor-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15080)

*Constraints*: *is-invocable-using*<F> is true[.](#func.wrap.ref.ctor-9.sentence-1)

[10](#func.wrap.ref.ctor-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15084)

*Mandates*: If is_pointer_v<F> || is_member_pointer_v<F> is true,
then f != nullptr is true[.](#func.wrap.ref.ctor-10.sentence-1)

[11](#func.wrap.ref.ctor-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15089)

*Effects*: Initializes*bound-entity* with a pointer to an unspecified object or
null pointer value, and*thunk-ptr* with the address of a function *thunk* such that*thunk*(*bound-entity*, *call-args*...) is expression-equivalent ([[defns.expression.equivalent]](defns.expression.equivalent "3.22 expression-equivalent")) toinvoke_r<R>(f, *call-args*...)[.](#func.wrap.ref.ctor-11.sentence-1)

[🔗](#lib:function_ref,constructor___)

`template<auto f, class U>
  constexpr function_ref(nontype_t<f>, U&& obj) noexcept;
`

[12](#func.wrap.ref.ctor-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15108)

Let T be remove_reference_t<U> andF be decltype(f)[.](#func.wrap.ref.ctor-12.sentence-1)

[13](#func.wrap.ref.ctor-13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15112)

*Constraints*: 

- [(13.1)](#func.wrap.ref.ctor-13.1)

is_rvalue_reference_v<U&&> is false, and

- [(13.2)](#func.wrap.ref.ctor-13.2)

*is-invocable-using*<F, cv T&> is true[.](#func.wrap.ref.ctor-13.sentence-1)

[14](#func.wrap.ref.ctor-14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15119)

*Mandates*: If is_pointer_v<F> || is_member_pointer_v<F> is true,
then f != nullptr is true[.](#func.wrap.ref.ctor-14.sentence-1)

[15](#func.wrap.ref.ctor-15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15124)

*Effects*: Initializes*bound-entity* with addressof(obj), and*thunk-ptr* with the address of a function *thunk* such that*thunk*(*bound-entity*, *call-args*...) is expression-equivalent ([[defns.expression.equivalent]](defns.expression.equivalent "3.22 expression-equivalent")) toinvoke_r<R>(f, static_cast<cv T&>(obj), *call-args*...)[.](#func.wrap.ref.ctor-15.sentence-1)

[🔗](#lib:function_ref,constructor____)

`template<auto f, class T>
  constexpr function_ref(nontype_t<f>, cv T* obj) noexcept;
`

[16](#func.wrap.ref.ctor-16)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15142)

Let F be decltype(f)[.](#func.wrap.ref.ctor-16.sentence-1)

[17](#func.wrap.ref.ctor-17)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15145)

*Constraints*: *is-invocable-using*<F, cv T*> is true[.](#func.wrap.ref.ctor-17.sentence-1)

[18](#func.wrap.ref.ctor-18)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15149)

*Mandates*: If is_pointer_v<F> || is_member_pointer_v<F> is true,
then f != nullptr is true[.](#func.wrap.ref.ctor-18.sentence-1)

[19](#func.wrap.ref.ctor-19)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15154)

*Preconditions*: If is_member_pointer_v<F> is true,obj is not a null pointer[.](#func.wrap.ref.ctor-19.sentence-1)

[20](#func.wrap.ref.ctor-20)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15159)

*Effects*: Initializes*bound-entity* with obj, and*thunk-ptr* with the address of a function *thunk* such that*thunk*(*bound-entity*, *call-args*...) is expression-equivalent ([[defns.expression.equivalent]](defns.expression.equivalent "3.22 expression-equivalent")) toinvoke_r<R>(f, obj, *call-args*...)[.](#func.wrap.ref.ctor-20.sentence-1)

[🔗](#lib:operator=,function_ref)

`template<class T> function_ref& operator=(T) = delete;
`

[21](#func.wrap.ref.ctor-21)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15176)

*Constraints*: 

- [(21.1)](#func.wrap.ref.ctor-21.1)

T is not the same type as function_ref,

- [(21.2)](#func.wrap.ref.ctor-21.2)

is_pointer_v<T> is false, and

- [(21.3)](#func.wrap.ref.ctor-21.3)

T is not a specialization of nontype_t[.](#func.wrap.ref.ctor-21.sentence-1)

#### [22.10.17.6.4](#func.wrap.ref.inv) Invocation [[func.wrap.ref.inv]](func.wrap.ref.inv)

[🔗](#lib:operator(),function_ref)

`R operator()(ArgTypes... args) const noexcept(noex);
`

[1](#func.wrap.ref.inv-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15193)

*Effects*: Equivalent to:return *thunk-ptr*(*bound-entity*, std​::​forward<ArgTypes>(args)...);

#### [22.10.17.6.5](#func.wrap.ref.deduct) Deduction guides [[func.wrap.ref.deduct]](func.wrap.ref.deduct)

[🔗](#func.wrap.ref.deduct-itemdecl:1)

`template<class F>
  function_ref(F*) -> function_ref<F>;
`

[1](#func.wrap.ref.deduct-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15207)

*Constraints*: is_function_v<F> is true[.](#func.wrap.ref.deduct-1.sentence-1)

[🔗](#func.wrap.ref.deduct-itemdecl:2)

`template<auto f>
  function_ref(nontype_t<f>) -> function_ref<see below>;
`

[2](#func.wrap.ref.deduct-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15218)

Let F be remove_pointer_t<decltype(f)>[.](#func.wrap.ref.deduct-2.sentence-1)

[3](#func.wrap.ref.deduct-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15221)

*Constraints*: is_function_v<F> is true[.](#func.wrap.ref.deduct-3.sentence-1)

[4](#func.wrap.ref.deduct-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15225)

*Remarks*: The deduced type is function_ref<F>[.](#func.wrap.ref.deduct-4.sentence-1)

[🔗](#func.wrap.ref.deduct-itemdecl:3)

`template<auto f, class T>
  function_ref(nontype_t<f>, T&&) -> function_ref<see below>;
`

[5](#func.wrap.ref.deduct-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15236)

Let F be decltype(f)[.](#func.wrap.ref.deduct-5.sentence-1)

[6](#func.wrap.ref.deduct-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15239)

*Constraints*: 

- [(6.1)](#func.wrap.ref.deduct-6.1)

F is of the formR(G​::​*)(A...) cv &opt noexcept(E) for a type G, or

- [(6.2)](#func.wrap.ref.deduct-6.2)

F is of the formM G​::​* for a type G and an object type M,
in which case
let R be invoke_result_t<F, T&>,A... be an empty pack, andE be false, or

- [(6.3)](#func.wrap.ref.deduct-6.3)

F is of the formR(*)(G, A...) noexcept(E) for a type G[.](#func.wrap.ref.deduct-6.sentence-1)

[7](#func.wrap.ref.deduct-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15259)

*Remarks*: The deduced type is function_ref<R(A...) noexcept(E)>[.](#func.wrap.ref.deduct-7.sentence-1)

### [22.10.18](#func.search) Searchers [[func.search]](func.search)

#### [22.10.18.1](#func.search.general) General [[func.search.general]](func.search.general)

[1](#func.search.general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15269)

Subclause [[func.search]](#func.search "22.10.18 Searchers") provides function object types ([function.objects]) for
operations that search for a sequence [pat_first, pat_last) in another
sequence [first, last) that is provided to the object's function call
operator[.](#func.search.general-1.sentence-1)

The first sequence (the pattern to be searched for) is provided to
the object's constructor, and the second (the sequence to be searched) is
provided to the function call operator[.](#func.search.general-1.sentence-2)

[2](#func.search.general-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15277)

Each specialization of a class template specified in [[func.search]](#func.search "22.10.18 Searchers") shall meet the [*Cpp17CopyConstructible*](utility.arg.requirements#:Cpp17CopyConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") and [*Cpp17CopyAssignable*](utility.arg.requirements#:Cpp17CopyAssignable "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements[.](#func.search.general-2.sentence-1)

Template parameters named

- [(2.1)](#func.search.general-2.1)

ForwardIterator,

- [(2.2)](#func.search.general-2.2)

ForwardIterator1,

- [(2.3)](#func.search.general-2.3)

ForwardIterator2,

- [(2.4)](#func.search.general-2.4)

RandomAccessIterator,

- [(2.5)](#func.search.general-2.5)

RandomAccessIterator1,

- [(2.6)](#func.search.general-2.6)

RandomAccessIterator2, and

- [(2.7)](#func.search.general-2.7)

BinaryPredicate

of templates specified in[[func.search]](#func.search "22.10.18 Searchers") shall meet the same requirements and semantics as
specified in [[algorithms.general]](algorithms.general "26.1 General")[.](#func.search.general-2.sentence-2)

Template parameters named Hash shall meet the [*Cpp17Hash*](hash.requirements#:Cpp17Hash "16.4.4.5 Cpp17Hash requirements [hash.requirements]") requirements (Table [37](hash.requirements#tab:cpp17.hash "Table 37: Cpp17Hash requirements"))[.](#func.search.general-2.sentence-3)

[3](#func.search.general-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15296)

The Boyer-Moore searcher implements the Boyer-Moore search algorithm[.](#func.search.general-3.sentence-1)

The Boyer-Moore-Horspool searcher implements the Boyer-Moore-Horspool search algorithm[.](#func.search.general-3.sentence-2)

In general, the Boyer-Moore searcher will use more memory and give better runtime performance than Boyer-Moore-Horspool[.](#func.search.general-3.sentence-3)

#### [22.10.18.2](#func.search.default) Class template default_searcher [[func.search.default]](func.search.default)

[🔗](#lib:default_searcher)

namespace std {template<class ForwardIterator1, class BinaryPredicate = equal_to<>>class default_searcher {public:constexpr default_searcher(ForwardIterator1 pat_first, ForwardIterator1 pat_last,
 BinaryPredicate pred = BinaryPredicate()); template<class ForwardIterator2>constexpr pair<ForwardIterator2, ForwardIterator2>operator()(ForwardIterator2 first, ForwardIterator2 last) const; private: ForwardIterator1 pat_first_; // *exposition only* ForwardIterator1 pat_last_; // *exposition only* BinaryPredicate pred_; // *exposition only*};}

[🔗](#lib:default_searcher,constructor)

`constexpr default_searcher(ForwardIterator1 pat_first, ForwardIterator1 pat_last,
                           BinaryPredicate pred = BinaryPredicate());
`

[1](#func.search.default-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15331)

*Effects*: Constructs a default_searcher object, initializing pat_first_ with pat_first, pat_last_ with pat_last, andpred_ with pred[.](#func.search.default-1.sentence-1)

[2](#func.search.default-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15338)

*Throws*: Any exception thrown by the copy constructor of BinaryPredicate orForwardIterator1[.](#func.search.default-2.sentence-1)

[🔗](#lib:operator(),default_searcher)

`template<class ForwardIterator2>
  constexpr pair<ForwardIterator2, ForwardIterator2>
    operator()(ForwardIterator2 first, ForwardIterator2 last) const;
`

[3](#func.search.default-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15352)

*Effects*: Returns a pair of iterators i and j such that

- [(3.1)](#func.search.default-3.1)

i == search(first, last, pat_first_, pat_last_, pred_), and

- [(3.2)](#func.search.default-3.2)

if i == last, then j == last,
otherwise j == next(i, distance(pat_first_, pat_last_))[.](#func.search.default-3.sentence-1)

#### [22.10.18.3](#func.search.bm) Class template boyer_moore_searcher [[func.search.bm]](func.search.bm)

[🔗](#lib:boyer_moore_searcher)

namespace std {template<class RandomAccessIterator1, class Hash = hash<typename iterator_traits<RandomAccessIterator1>::value_type>, class BinaryPredicate = equal_to<>>class boyer_moore_searcher {public: boyer_moore_searcher(RandomAccessIterator1 pat_first,
 RandomAccessIterator1 pat_last,
 Hash hf = Hash(),
 BinaryPredicate pred = BinaryPredicate()); template<class RandomAccessIterator2> pair<RandomAccessIterator2, RandomAccessIterator2>operator()(RandomAccessIterator2 first, RandomAccessIterator2 last) const; private: RandomAccessIterator1 pat_first_; // *exposition only* RandomAccessIterator1 pat_last_; // *exposition only* Hash hash_; // *exposition only* BinaryPredicate pred_; // *exposition only*};}

[🔗](#lib:boyer_moore_searcher,constructor)

`boyer_moore_searcher(RandomAccessIterator1 pat_first,
                     RandomAccessIterator1 pat_last,
                     Hash hf = Hash(),
                     BinaryPredicate pred = BinaryPredicate());
`

[1](#func.search.bm-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15399)

*Preconditions*: The value type of RandomAccessIterator1 meets
the [*Cpp17DefaultConstructible*](utility.arg.requirements#:Cpp17DefaultConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]"),[*Cpp17CopyConstructible*](utility.arg.requirements#:Cpp17CopyConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]"), and[*Cpp17CopyAssignable*](utility.arg.requirements#:Cpp17CopyAssignable "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements[.](#func.search.bm-1.sentence-1)

[2](#func.search.bm-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15406)

Let V be iterator_traits<RandomAccessIterator1>​::​value_type[.](#func.search.bm-2.sentence-1)

For any two values A and B of type V,
if pred(A, B) == true, then hf(A) == hf(B) is true[.](#func.search.bm-2.sentence-2)

[3](#func.search.bm-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15411)

*Effects*: Initializespat_first_ with pat_first,pat_last_ with pat_last,hash_ with hf, andpred_ with pred[.](#func.search.bm-3.sentence-1)

[4](#func.search.bm-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15420)

*Throws*: Any exception thrown by the copy constructor of RandomAccessIterator1,
or by the default constructor, copy constructor, or the copy assignment operator of the value type of RandomAccessIterator1,
or the copy constructor or operator() of BinaryPredicate or Hash[.](#func.search.bm-4.sentence-1)

May throw bad_alloc if additional memory needed for internal data structures cannot be allocated[.](#func.search.bm-4.sentence-2)

[🔗](#lib:operator(),boyer_moore_searcher)

`template<class RandomAccessIterator2>
  pair<RandomAccessIterator2, RandomAccessIterator2>
    operator()(RandomAccessIterator2 first, RandomAccessIterator2 last) const;
`

[5](#func.search.bm-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15436)

*Mandates*: RandomAccessIterator1 and RandomAccessIterator2 have the same value type[.](#func.search.bm-5.sentence-1)

[6](#func.search.bm-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15441)

*Effects*: Finds a subsequence of equal values in a sequence[.](#func.search.bm-6.sentence-1)

[7](#func.search.bm-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15445)

*Returns*: A pair of iterators i and j such that

- [(7.1)](#func.search.bm-7.1)

i is the first iterator
in the range [first, last - (pat_last_ - pat_first_)) such that
for every non-negative integer n less than pat_last_ - pat_first_ the following condition holds:pred(*(i + n), *(pat_first_ + n)) != false, and

- [(7.2)](#func.search.bm-7.2)

j == next(i, distance(pat_first_, pat_last_))[.](#func.search.bm-7.sentence-1)

Returns make_pair(first, first) if [pat_first_, pat_last_) is empty,
otherwise returns make_pair(last, last) if no such iterator is found[.](#func.search.bm-7.sentence-2)

[8](#func.search.bm-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15459)

*Complexity*: At most (last - first) * (pat_last_ - pat_first_) applications of the predicate[.](#func.search.bm-8.sentence-1)

#### [22.10.18.4](#func.search.bmh) Class template boyer_moore_horspool_searcher [[func.search.bmh]](func.search.bmh)

[🔗](#lib:boyer_moore_horspool_searcher)

namespace std {template<class RandomAccessIterator1, class Hash = hash<typename iterator_traits<RandomAccessIterator1>::value_type>, class BinaryPredicate = equal_to<>>class boyer_moore_horspool_searcher {public: boyer_moore_horspool_searcher(RandomAccessIterator1 pat_first,
 RandomAccessIterator1 pat_last,
 Hash hf = Hash(),
 BinaryPredicate pred = BinaryPredicate()); template<class RandomAccessIterator2> pair<RandomAccessIterator2, RandomAccessIterator2>operator()(RandomAccessIterator2 first, RandomAccessIterator2 last) const; private: RandomAccessIterator1 pat_first_; // *exposition only* RandomAccessIterator1 pat_last_; // *exposition only* Hash hash_; // *exposition only* BinaryPredicate pred_; // *exposition only*};}

[🔗](#lib:boyer_moore_horspool_searcher,constructor)

`boyer_moore_horspool_searcher(RandomAccessIterator1 pat_first,
                              RandomAccessIterator1 pat_last,
                              Hash hf = Hash(),
                              BinaryPredicate pred = BinaryPredicate());
`

[1](#func.search.bmh-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15501)

*Preconditions*: The value type of RandomAccessIterator1 meets the [*Cpp17DefaultConstructible*](utility.arg.requirements#:Cpp17DefaultConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]"),[*Cpp17CopyConstructible*](utility.arg.requirements#:Cpp17CopyConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]"), and [*Cpp17CopyAssignable*](utility.arg.requirements#:Cpp17CopyAssignable "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements[.](#func.search.bmh-1.sentence-1)

[2](#func.search.bmh-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15506)

Let V be iterator_traits<RandomAccessIterator1>​::​value_type[.](#func.search.bmh-2.sentence-1)

For any two values A and B of type V,
if pred(A, B) == true, then hf(A) == hf(B) is true[.](#func.search.bmh-2.sentence-2)

[3](#func.search.bmh-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15511)

*Effects*: Initializespat_first_ with pat_first,pat_last_ with pat_last,hash_ with hf, andpred_ with pred[.](#func.search.bmh-3.sentence-1)

[4](#func.search.bmh-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15520)

*Throws*: Any exception thrown by the copy constructor of RandomAccessIterator1,
or by the default constructor, copy constructor, or the copy assignment operator of the value type of RandomAccessIterator1,
or the copy constructor or operator() of BinaryPredicate or Hash[.](#func.search.bmh-4.sentence-1)

May throw bad_alloc if additional memory needed for internal data structures cannot be allocated[.](#func.search.bmh-4.sentence-2)

[🔗](#lib:operator(),boyer_moore_horspool_searcher)

`template<class RandomAccessIterator2>
  pair<RandomAccessIterator2, RandomAccessIterator2>
    operator()(RandomAccessIterator2 first, RandomAccessIterator2 last) const;
`

[5](#func.search.bmh-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15536)

*Mandates*: RandomAccessIterator1 and RandomAccessIterator2 have the same value type[.](#func.search.bmh-5.sentence-1)

[6](#func.search.bmh-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15541)

*Effects*: Finds a subsequence of equal values in a sequence[.](#func.search.bmh-6.sentence-1)

[7](#func.search.bmh-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15545)

*Returns*: A pair of iterators i and j such that

- [(7.1)](#func.search.bmh-7.1)

i is the first iterator in the range
[first, last - (pat_last_ - pat_first_)) such that
for every non-negative integer n less than pat_last_ - pat_first_ the following condition holds:pred(*(i + n), *(pat_first_ + n)) != false, and

- [(7.2)](#func.search.bmh-7.2)

j == next(i, distance(pat_first_, pat_last_))[.](#func.search.bmh-7.sentence-1)

Returns make_pair(first, first) if [pat_first_, pat_last_) is empty,
otherwise returns make_pair(last, last) if no such iterator is found[.](#func.search.bmh-7.sentence-2)

[8](#func.search.bmh-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15559)

*Complexity*: At most (last - first) * (pat_last_ - pat_first_) applications of the predicate[.](#func.search.bmh-8.sentence-1)

### [22.10.19](#unord.hash) Class template hash [[unord.hash]](unord.hash)

[1](#unord.hash-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15566)

The unordered associative containers defined in [[unord]](unord "23.5 Unordered associative containers") use
specializations of the class template hash ([[functional.syn]](#functional.syn "22.10.2 Header <functional> synopsis"))
as the default hash function[.](#unord.hash-1.sentence-1)

[2](#unord.hash-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15573)

Each specialization of hash is either enabled or disabled,
as described below[.](#unord.hash-2.sentence-1)

[*Note [1](#unord.hash-note-1)*:

Enabled specializations meet the [*Cpp17Hash*](hash.requirements#:Cpp17Hash "16.4.4.5 Cpp17Hash requirements [hash.requirements]") requirements, and
disabled specializations do not[.](#unord.hash-2.sentence-2)

— *end note*]

Each header that declares the template hash provides enabled specializations of hash for nullptr_t and
all cv-unqualified arithmetic, enumeration, and pointer types[.](#unord.hash-2.sentence-3)

For any type Key for which neither the library nor the user provides
an explicit or partial specialization of the class template hash,hash<Key> is disabled[.](#unord.hash-2.sentence-4)

[3](#unord.hash-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15587)

If the library provides an explicit or partial specialization of hash<Key>,
that specialization is enabled except as noted otherwise,
and its member functions are noexcept except as noted otherwise[.](#unord.hash-3.sentence-1)

[4](#unord.hash-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15592)

If H is a disabled specialization of hash,
these values are false:is_default_constructible_v<H>,is_copy_constructible_v<H>,is_move_constructible_v<H>,is_copy_assignable_v<H>, andis_move_assignable_v<H>[.](#unord.hash-4.sentence-1)

Disabled specializations of hash are not function object types ([function.objects])[.](#unord.hash-4.sentence-2)

[*Note [2](#unord.hash-note-2)*:

This means that the specialization of hash exists, but
any attempts to use it as a [*Cpp17Hash*](hash.requirements#:Cpp17Hash "16.4.4.5 Cpp17Hash requirements [hash.requirements]") will be ill-formed[.](#unord.hash-4.sentence-3)

— *end note*]

[5](#unord.hash-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L15607)

An enabled specialization hash<Key> will:

- [(5.1)](#unord.hash-5.1)

meet the [*Cpp17Hash*](hash.requirements#:Cpp17Hash "16.4.4.5 Cpp17Hash requirements [hash.requirements]") requirements (Table [37](hash.requirements#tab:cpp17.hash "Table 37: Cpp17Hash requirements")),
with Key as the function
call argument type, the [*Cpp17DefaultConstructible*](utility.arg.requirements#:Cpp17DefaultConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements (Table [30](utility.arg.requirements#tab:cpp17.defaultconstructible "Table 30: Cpp17DefaultConstructible requirements")),
the [*Cpp17CopyAssignable*](utility.arg.requirements#:Cpp17CopyAssignable "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements (Table [34](utility.arg.requirements#tab:cpp17.copyassignable "Table 34: Cpp17CopyAssignable requirements (in addition to Cpp17MoveAssignable)")),
the [*Cpp17Swappable*](swappable.requirements#:Cpp17Swappable "16.4.4.3 Swappable requirements [swappable.requirements]") requirements ([[swappable.requirements]](swappable.requirements "16.4.4.3 Swappable requirements")),

- [(5.2)](#unord.hash-5.2)

meet the requirement that if k1 == k2 is true, h(k1) == h(k2) is
also true, where h is an object of type hash<Key> and k1 and k2 are objects of type Key;

- [(5.3)](#unord.hash-5.3)

meet the requirement that the expression h(k), where h is an object of type hash<Key> and k is an object of typeKey, shall not throw an exception unless hash<Key> is a
program-defined specialization[.](#unord.hash-5.sentence-1)